Abstract: In the North China Plain (NCP), winter wheat and summer maize double cropping consumed 800~850 mm of water annually. About 25%~30% of the water loss was due to soil evaporation. Reducing soil water evaporation was an effective water-saving measure in this seriously water-scarce region. Straw mulch has been proved to be one of the effective methods used to reduce soil evaporation. Double cropping of winter wheat and summer maize in the NCP was effective in producing abundant straw for mulching. With the mechanization of winter wheat harvest, winter wheat straw has been successfully used to mulch maize; significantly reducing crop water use. However, summer maize straw mulch on the soil surface has created unfavorable winter wheat growth conditions resulting in yield reduction. Because straw mulch reduced winter wheat yield, its practical application was limited. Decrease in spike number was main reason for the yield reduction of winter, and the 1000-grain weight decrease was the second reason. Soil surface straw mulch reduced maximum root zone temperature during daytime and increased minimum temperature during nighttime, thereby reducing daily temperature range. Changes in root zone temperature under straw mulch delay growth period and reduced yield. This paper summarized the causes and mechanisms of winter wheat yield reduction under straw mulch in the NCP. The results were vital in finding technical solutions to the negative effects of straw mulch on winter wheat. This enhanced the practical application of straw mulch to save irrigation water in the NCP.