Mass flow measurement during the crop harvest process is one of important ways of data collecting for precision agriculture. In this research, a method for detecting seedcotton mass flow rate was proposed. The principle of the method was based on the fact that the dielectric constant of air/seedcotton mixture between two parallel plates increased with seedcotton mass flow rate. In order to decrease the influence of environmental disturbance, a differential capacitive sensor was designed to perform the seedcotton mass flow rate measurement. One kind of seedcotton named Xinluzao43 was used as sample to study the influence of cotton moisture content on the output capacitance value of mass flow sensors, and the relationship of cotton mass flow and sensor output capacitance. Experiment results showed that, the relationship among cotton moisture content, mass flow rate and capacitance value could be described by a bivariate regression model. The change of ambient temperature and breed had no significant relationship with the sensor output. The average relative error of seedcotton mass flow sensor was 5.16%. 