By analyzing the integral constant C1 of Alfven solitons, the condition of Alfven Gaussian wave-packets evolvement to solitary waves is put forward as C1≤0. This hypothesis is also confirmed by numerical simulation results. Furthermore,simulation results show that the number of evolving solitons does not depend only on integral constant C0 or C1. The conclusions in Refs. ［1］ and ［2］ result from the shorter evolving time. Comparing with other papers, we find that C0≤0 can be used as a criterion of Alfven wave modulation instability.