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Finding out the laws of crop irrigation demand and the main control factors in North China Plain (NCP) is an important basis for rationally formulating water resources planning and alleviating groundwater overexploitation. Based on the daily meteorological data of 60 weather stations in the past 50 years,the Penman-Monteith formula recommended by FAO was used to calculate crop water demand, and the impact of precipitation on the spatiotemporal characteristics of irrigation demand of winter wheat and summer maize as the main grain crops was analyzed. The results showed that at the years with abundant precipitation (25%), the distribution of the irrigation water demand index IRI for winter wheat in NCP was mainly in the high irrigation demand with IRI from 0.50 to 0.75, and that of summer maize was mainly in medium irrigation demand with IRI of 0.25~0.50, accounting for 92% and 86% of the NCP respectively. At the years with normal precipitation (50%), the IRI for winter wheat and summer maize was mainly in the range of above 0.75 and 0.25~0.50, accounting for 56% and 100% of the NCP respectively. At the years with low precipitation (75%), the IRI was mainly in extremely high irrigation demand for winter wheat, accounting for 97% of the NCP, and in high irrigation demand for summer maize. Precipitation was the main factor affecting IRI. With the increase of precipitation, the IRI showed a linear downward trend, but there were significant differences in sensitivity to precipitation changes for different locations of the NCP. For winter wheat, most sensitive regions were Yanshan piedmont plain, Taihang piedmont-Jizhong plain, Central-Jizhong plain and Coastal plain, and for every 100mm increases in precipitation, the IRI was decreased by 0.14, while summer maize was in the Middle-Jizhong plain, and for every 100mm increases in precipitation, the IRI was decreased by 0.26. The IRI of winter wheat and summer maize all obeyed normal distribution, but with the change of precipitation, the distribution parameters were changed obviously.