Abstract: Studies indicate that crab grass has a growth period of 62—111 days and needs>0℃ accumulated temperature of 1380℃–2176℃. Grab grass has a strict demands on season and temperature at vegetative stage, but is insensitive to these conditions at reproductive stage. The length of growth period is mainly determined by that of the vegetative growth stage.Seed storage conditions have great bearings on the maintenance and release of seed dormancy. The release of seed dormancy is fastest under storage conditions of constant temperature.The minimum temperature for seed germination is 15℃. The effective accumulated temperature for the germination of 60% of the seeds is 555.13℃. The relationship between seed germination rate(y) and temperature(x) can be described by the following formula: y = - 103.722 + 15.133x- 0.288x2. The germination of seeds is insensitive to light. The results showed that the seeds buried to a depth of 7 cm generally emerged, and the maximum emergence rate occurred for the seeds buried in the depths of 1—4cm.Crab grass has a great reproductive capacity. Our results showed that almost all cutting nodals with fibrous roots and buds could develop to new plants with the exception of ear stems.The results of path analysis indicate that crab grass is a main weed for summer grain crops in Hebei Province. The relationship between the loss of crop yield(y, %)and the density(x,Plants/m2)of crab grasses could be expressed by the formula: y = 0.1532xl.5574(x≤20Plants/m2),or y = - 18.400 + 11.315lnx (x>20 Plants/m2). And the relationship between the loss of crop yield(y, %)and the days of coexistence(x) of crab grass with summer corn was formulated as: y=54.5956/ 1 + 89.7962e-0.1184x Based on our calculations, the critical density for crab grass in summer corn field in Shijiazhuang region is 5.8 Plants/m2.