Abstract: in order to study the expansion law of the saturated water coal racture under the infitaion of the liuid nirogen.,the cubic raw cal samples taken fromWangyingziMine,ater the vacuum saturation,the lquid ntirogen leaching experiment was conducted. The sound velocity of the coal sample was tested before and afitthe liquid nitrogen leaching and the representative crack area variation rate and coal sample surface micro structure variation were tested. Under th liquid niragen roe,the frost swelling pressure of the ice was derived from the water phase in the saturated water cracks and the mechanics crterion of the saurated warer crack expanded was established under th liquid ntrogen role.The results showed that 1 under infitration h licuid ntrogen.the crack water in the coal rnstomed toice,the woume was swelled,the cracks within the coal were expanded.,the representative crack area was inreased,th ultrasonit velocity was rediced and the crack dersty was ineased. The micro structure of the coal was highly varied. 2 The mechanics crterion estabished could accurat ely predict the crack expansion of the cal sample. withthe length ratio of long axis and minor axis decreased ,the frost swellig pressure would be inreased and with the waer fiov coerficsent inrea sed,thelnear of the frostswelling would be decreased.