Variable rate fertilization has the advantages of improving fertilizer-utilization efficiency, protecting ecological environment as well as saving agricultural production cost. But it has not been widely applied yet, besides it is hard for getting the prescription figure, lacking closed-loop detection is another major reason. Closed-loop control is one critical step towards realizing the variable rate fertilization, compared with the indirect measurement which monitors the axis speed, it is more accurate by monitoring the real-time mass flow rate. If there existed the fertilizer caking that blocked the fertilizer apparatus, it is useless for monitoring the axis speed. Based on the electrostatic induction theory, a sensor that could monitor the mass flow rate of granular fertilizer was designed. Owing to the frictions and collisions between the granular fertilizer and the air, the granular fertilizer and the fertilizer tube, as well as the frictions and collisions among the granular fertilizer themselves, therefore, the granular fertilizers would carry a certain amount of electric charges. One ring electrode was designed to detect the strength of the electric charges, subsequently, a corresponding current amplifying circuit was utilized to export the induced current. The real-time mass flow rate could be obtained by calibrating the relationship between it and the induced current. One test-bed was established in order to finish the task, the test-bed mainly included one dynamic signal acquisition system, one fertilizer box, one current amplifier and the sensor. Large granular urea (CO(NH2)2), superphosphate (Ca(H2PO4)2·H2O) as well as potassium chloride (KCl) were chosen as the research objects, their mean bulk densities were 0.7g/cm3, 1.2g/cm3 and 1.1g/cm3, respectively. According to the physical parameters of the fertilizer apparatus, the approximate target mass flow rates could be acquired by adjusting the axis speeds, and the target mass flow rates were ranged from 3g/s to 15g/s with increment of 1g/s. With respect to each mass flow rate, four replicates were conducted. Each replicate lasted for 30 s, and the fertilizer apparatus was started at the same time with the signal acquisition system. The average induced current and average mass flow rate were used to establish related regression equations, thus the real-time mass flow rate could be got by interpolation method. Subsequently, totally 25 experiments of each fertilizer were conducted to study the measurement accuracy, the targeted mass flow rates for each experiment were composited by five randomized mass flow rates, and each mass flow rate would last for 6s. The real mass during the 30s would be weighed by balance, while the detective mass was calculated by integrating the mass flow rate and time curves. The experimental results showed that there was no significant difference between the detective mass and the real mass (P>0.1), and the detective errors for large granular urea, superphosphate as well as potassium chloride were 3.9%, 5.1% and 5.9%, the corresponding standard errors were 5.21, 7.98 and 11.29. In regards to the granular fertilizer, the larger of the superficial area was, the easier of getting induced charge and saturation was. Consequently, the induced current would be larger, and the detective ring electrode was more sensitive on relative larger induced current. The mean diameters of the large granular urea, superphosphate and potassium chloride were 4.43mm, 2.77mm and 2.03mm, so the mean superficial areas should be in the same order, conclusions that generated from the research results showed that the detective error was increased along with the decrease of granular dimensions. SPSS 22.0 was used to handle further statistical analysis, the error distributions of three fertilizers were accorded with normal distribution, which meant the errors would be within ±6% under most of circumstances, the mathematical expectations of the detective errors were 3.74%，4.93% and 5.22% for large granular urea, superphosphate and potassium chloride respectively. The mass flow rate sensor that used for granular fertilizer could satisfy the requirements of real-time detection, the test-bed that designed could provide references for the research of variable rate fertilization.