Abstract: Research has been undertaken using the validated Agricultural Production System Simulator Model (APSIM) to explore the synergy effects of water and fertilizer supply on grain yields of crops under different tillage practices in the loess hilly region of Dingxi City, Gansu Province. The model was used to simulate yields of rotational systems of wheat (Triticum aestivum) and pea (Pisum sativum) under three different tillage practices: traditional tillage (T), no tillage (NT) and no tillage mulched with straw within 35 years (NTS). Multiple regression analysis was used to investigate the synergy effects of precipitation at the growing stage (X₃), fallow period (X₂) and when nitrogen was applied (X₁). Results showed that the order of relevance to grain yields was X₃>X₂>X₁. For X₃ and X₂, the contribution rate differed in the order of NTS＞NT＞T. Nitrogen application has an oppositive U-shaped quadratic relation with wheat and pea yield. The optimum nitrogen quantities were 65.0, 65.5 and 44.5 kg/ha for spring wheat and 17.9, 18.5 and 23.8 kg/ha for peas under T, NT and NTS respectively. The contribution of nitrogen to the grain yields of each rotational system followed the order NTS＞NT＞T for spring wheat and NTS＞NT＞T for peas. Precipitation was therefore the key factor for the grain yields of spring wheat and pea in this region, especially in the case of NTS. Nitrogen application had different effects on yield under the three different tillage practices.