The fast, real-time, non-destructive and quantitative monitoring of plant nitrogen status and precise regulation of nitrogen fertilizer has important practical significance for the development of double cropping rice production with higher yield, better grain quality, higher economic efficiency and more ecological safety. The objective of this study was to establish regulation model of nitrogen fertilizer for double cropping rice based on the portable apparatus for crop growth monitoring and diagnosis, which was a multi-spectral sensor containing 810 and 720 nm detection lens and structurally was divided into up-sensor and down-sensor, then recommend nitrogen topdressing rate, so as to achieve precise management of nitrogen topdressing. The spectra monitoring model of leaf area index (LAI) was established based on the datasets of field experiment with different plant type cultivars and nitrogen application rates. The nitrogen topdressing rate were calculated with the newly developed regulation of nitrogen fertilizer model and high yield cultivation experience with LAI, which was real-time estimated from the differential vegetation index (DVI) at jointing stage. Then the recommend nitrogen topdressing rate and grain yield were compared with those of farmer's nitrogen management treatment. Canopy DVI (810, 720) was well positive correlated with LAI at key development stages (tillering stage, jointing stage, booting stage, heading stage and filling stage) of the double cropping rice, and the linear function better fitted the relation than other functions. The models were validated using independent field experiment datasets, involving different plant type cultivars and nitrogen application rates, the prediction effect of model for single stage was better than the whole stage, especially at jointing stage. The coefficient of determination (R2) of spectral monitoring model for LAI in early rice and late rice was 0.880 6 and 0.878 8, respectively. The root mean square error (RMSE), relative root mean square error (RRMSE), correlation coefficient (r) of prediction of LAI in early rice and late rice was 0.30 and 0.25, 7.28% and 6.18%, 0.923 2 and 0.926 9, respectively. The results of recommend nitrogen topdressing based on regulation model of nitrogen fertilizer indicated that the panicle nitrogen rate of compact plant type cultivar was higher than that of loose plant type cultivar. Compared with the farmer's nitrogen management treatment, the regulation treatments based on regulation model of nitrogen fertilizer obtained equivalent or high grain yield with reduced nitrogen application rate 6.58 kg/hm2, while nitrogen agronomic efficiency, net income and yield-cost ratio was improved 0.82 percent point, 103 yuan/hm2 and 0.9, respectively. Compared with the normal method, the real-time topdressing regulation method based on portable apparatus for crop growth monitoring and diagnosis can reduce nitrogen application rate, raise nitrogen agronomic efficiency and higher economic benefits are also achieved on the premise of grain yield, which has a potential to be widely applied for precise nitrogen management and high yield cultivation in double cropping rice production.