Abstract: Based on the theory of fracture mechanics，analyzed and calculated the quasi-static stress field in the coal and the model of fracture propagation driven by gases of explosion products. Then proved the length of crack propagation caused by blasting loads. Finally through a blasting test of simulation coal，the results showed that the length of crack propagation was inversely proportional to both longitudinal wave velocity and compressive strength，and was directly proportional to porosity. The crack number and the crack length in the case of stemming of blast holes were respectively 1.5~3 times to that in the case of unblocked blast holes，verifying that in the process of coal blasting，the quasi-static stress of detonation gas was the main reason in the crack propagation and the blasting broken.

Key words: Detonation gas, Coal blasting, Quasi-static stress, Crack propagate