Abstract: In order to improve the performance of the variable rate fertilizer applicator’s control system, a control system for a variable rate fertilizer applicator based on the CPLD (complex programmable logic device) was designed and applied in field. Under automatic mode, the system judged the position and the ground speed for the variable rate fertilizer applicator by means of collecting GPS signals. At manual mode, the system judged the ground speed by speed sensor. This system controlled the fertilizer output through combination of a ground speed of fertilizer applicator and prescription data which were given by the Expert System according to the nutrient levels in soil and the theory of yield goal. This system has been applied in the field covering 72 hm2 totally in the past 4 years. The error of this system was less than 6%, when the fertilizer sowing amount was 150-600 kg/hm2.