It is important to detect grain moisture content for grain yield monitor, grain accurate possession and precision agriculture. It limits the accuracy and stability of grain moisture on-line detection for the dynamic condition of combine harvester.A grain moisture on-line detection device on combine harvester was developed based on the principle of dielectric property. The dynamic constant volume sampling mechanism was proposed. With the help of the sampling mechanism, a method of dynamic continuous sampling and static interval measurement was also proposed. It could detect grain moisture on the operation condition of combine harvester. The device consisted of mechanical dynamic sampling part, motor control module, sensor module, data acquisition module, GPS module and a display terminal. Sensor module included moisture sensor, temperature sensor and grain height level sensor. In June 2020, experiments were carried out at Beijing Xiaotangshan National Demonstration Station of Precision Agriculture. Experimental results showed that under the static condition, grain moisture on-line detection error was within 3%. Grain moisture detection model was established integrated of permittivity and grain temperature. The dynamic experiment showed that the correlation coefficient between the predicted grain moisture value and that of standard value was 0.92. The relevant error was less than 5%. It could satisfy the practice need.