Abstract: the theoretical analysis to local earthquake velocity magnitude formula and observational test shows that it is improper to substitute the maximum ground motion displacement with TV_max2π(V_max is maximum ground motion velocity;T is period) in determination of local earthquake magnitude.The magnitude determined with this method is generally smaller than the magnitude determined with analog displacement,and there is no simple relationship between the two magnitudes.The result to determine magnitude with (TV/2π)_max instead of the maximum ground displacement is correct,but it is not convenient in practice.Through integral to change velocity digital records into surface displacement,the magnitude we got is consistent with the magnitude from analog displacement.