The high power pulse laser energy spread is researched by statistic method, the energy spread is described in this paper. Supposing the output energy spread be known, the relation between the average energy and the probability of that which output energy is bigger than a given value is also described. Supposing the injected energy and parameters of optical elements obey normal distribution, basing on the multi-pass amplifier system where the main amplifier has 9 slabs and the booster has 5 slabs, the output energy spread about 1ns exponential injected pulse is simulated．