Based on the results of field fertilization experiments of rice conducted in Zhejiang Province, China, a decision support system for Optimization of Nitrogen to Irrigated Rice (ONIR) was developed, calibrated and validated. The system included NuDSS (the nutrient decision support system) and N Splitting Application Model (NSAM). NuDSS was a recently developed software for irrigated rice based on QUEFTS and SSNM, which was used to optimize nutrient management for rice varieties. According to NuDSS, the optimal fertilizer N requirement for the target yields could be calculated. We developed NSAM that could make sure N splitting application which based on the rhythm of growth and development of rice and nutrient balance. The results showed that the observed and simulated values of NA and DMA at different stages and grain yields at different N treatments fell near the 1:1 lines in the main. Values of α, R2 were in the range of 0.847 ~ 1.034 and 0.85 ~ 0.99, respectively. All P(t*) were larger than 0.05, yields absolute RMSE varied from 1 to 110 kg/hm2 and normalized RMSE varied from 9% - 24%. The attainable target yields were set to 70 to 75 percents of their potential yields. The attainable targets of late rice and middle rice in Zhejiang were 7.3 and 8.3 t/hm2 according to the potential yield simulated with WOFOST. On reasonable yield targets of late rice and middle rice in Zhejiang, the optimal fertilizer N requirement recommended by ONIR was about 120 kg/hm2 with splitting fractions of 0.4∶0.2∶0.4∶0 or 0.4∶0.2∶0.2∶0.2 and 150 kg/hm2 with splitting fractions of 0.4∶0.2∶0.2∶0 or 0.4∶0.2∶0.2∶0.2 as basal, at early tillering, panicle initiation and flowering stages, respectively. N application was only about 70% of the fertilizer N application under current farmers’ practice. N input could be reduced in some degree without yield loss if rice N management practice were improved in Jinhua of Zhejiang Province.