The technology of variable topdressing based on canopy spectral reflectance information is an effective way to realize the scientific and reasonable application of nitrogen fertilizer. In order to improve the precision of spectrum information and topdressing control system, the arrangement of spectrum sensor and the optimization of system control method were carried out. Finally, the field experiment of topdressing was carried out. The comparison test of upright mode and distributed arrangement mode showed that the distributed arrangement of spectrum sensor was better than that of upright mode. The average value of NDVI was increased by 6.4%, and the variance was reduced by 0.038. NDVI acquisition data was filtered by sliding window mean filtering algorithm. The sliding window size was 15 and the mean square error was 0.0079. The system response test showed that the average response time of the system was 1.5s, the absolute value of the average steady-state error was 0.775r/min, and the average overshoot was 10.6%. The system had a high control accuracy in the range of working speed of fertilizer wheel. The results of field fertilization control showed that the average relative error between theoretical rotation speed and monitoring rotation speed was 3.35%, which could achieve the goal of precision fertilization.