To improve the monitoring accuracy of the crop nitrogen sensor, this paper systematically investigated the optical system parameters affecting the monitoring accuracy of the monitor by analyzing principle of the sensor and the optical system design scheme, and designed a scheme of calibrating optical system parameters which included calibration of single photocell, multi-element combination and integrated optical system. Besides, the parameters calibration of optical system was further conducted by laboratory experiment. The data of rice field experiments were used to validate the effectiveness of the parameter calibration scheme of the optical system. The results indicate that the determination coefficient between normalized difference vegetation index (NDVI) and nitrogen content of rice leaves increases to 0.734 from originally 0.651. That is, the monitoring performance improves significantly after performing system calibration.