Qinghai Xizang Plateau is a high yield wheat area, and the highest yield of wheat can reach 15 2t/ha. Reasons accountable for high yield according to traditional concept are: (a)plentiful solar radiation; (b)great temperature range; (c)high intensity of photosynthesis of wheat; and (d)low consumption rate of respiration, which are thoughyt to be favorable for more accumulation of assimilation pro ducts in this area. However, experimental studies show that the mean rate of dry matters accumulation of wheat is a slightly lower than plain areas and traditional points can not explain this explicitly. Actually, because of low CO2 density in the plateau region, the efficiency of wheat leaf photosynthesis is just about 2/3 that of plain areas. Yet the high solar radiation and advantageous wheat community structure can make up deficiency of CO2 to some extent, so the mean rate of accumulation in the plateau region is just a slightly lower than plain areas. On the other hand, the annual average temperature is low in the plateau region which can elongate the duration of wheat growth greatly. So wheat has long time to take advantage of all the agro ecological conditions. The two factors, a slightly lower rate of dry matters accumulation and much longer duration of growing season, decide high product of dry matters in the plateau region. High harvest index wheat in the plateau region can get unusual high yield product.