Heterogeneity for response of plant populations to soil moisture is the environmental basis of their pattern formation and evolution. Melica przewalskyi is rhizomatous clonal grass with strange taste and not edible for grazing animals. With the infuluence of climate change and overgrazing, in the northern slope of Qilian Mountains, the Melica przewalskyi gradually formed a great number of single dominant species patches due to their biological characteristics and the adaptbility to adversity.Therefore, it is very important to understand their biologic characteristic of population distribution, distribution patterns and the soil moisture, and study the change of their population characteristic and soil moisture and the mechanism in the process of patch formation and recession. It is critical for revealing the plasticity mechanism of soil moisture and understanding the conspiracy relationship between the plant and the environment. The experiments were conducted in Yue ya ai grassland (38°47'15.8″N,99° 43' 35.9″E)which is located in the upper reaches of Shiyang river in Sunan country Gansu province, China. In early August,2010, three different patches in the different stage, such as formation stage, diffusion stage, stable stage and recession stage respectively were selected in the study area. Square quad rat was made using red chopstick and sample line, the patches were places into quad rat, the quadrats center was founded by diagonal of 4 fixed vertice, thus red chopsticks were used to state the quadrat center position. sampling points were set from centre to four directions, thesampling point on each direction was selected at least 50 cm, adjacency grid sample in each point was used, the coverage was measured using the needle puncturing method in 50 cm × 50 cm sample box and tested the density of Melica przewalskyi in the same sample box by counting, measuring the natural height by tape repeat six, recording the relative position of each plant and determine the moisture content of 0-20 cm by oven drying method. the measured value L(d) and the value of upper and lower envelopes were calculated by the VisualFortran6.5 based on the coordinate getting by GetData Graph Digitizer2.22, Origin7.5 was used as the plot software, this plot in the lower left corner coordinate origin to the intersection, using the step of 5 cm, the number of Monte_Carlo stochastic simulation is 20 times, dashed line are the 95% confidence envelops for the same process. The results showed that:Melica przewalskyi population spatial pattern transitted from clumped to random distribution at four stages, soil moisture tended to increase from center to edge of patch; Melica przewalskyi patch diffused to the high soil moisture with concentric circles way during the formation stage, diffused stage and stable stage, and the radius of patch expand from 0.5 m to 1.5 m, the population height, density and coverage were decreased from center to edge of patch; And then the center of patch appeared naked at recessionary stage, the height, density and coverage increased progressively from center to edge of patch in concentric circles way. The spatial pattern and main biological characteristics showed different correspondence relationship with soil moiture at different stages.