The theory of minimum cross entropy spctrum was briefly introduced. One experimental data was used to examine the program of minimum cross entropy spectrum,and we used the method of minimum cross entropy spectrum to analvse water level data of rock blast.Besides the day change of 0.98Hz,the results show that there would appear high frequency of 1.2Hz several davs before some rock blasts. This result can not be obtained by ordinary methods. The new method provides a new path of rock blast forecast.