For phasor measurement unit（PMU） can't be installed in all buses of a power system， how to analyze power system transient stability with little measurements is an urgent problem to be solved. This paper proposes a method of applying the equal area criterion（EAC） to multi-machine system transient stability assessment directly: the machines with the most leading angle and the most lagging angle are selected， and by fitting their relative power angle curve， EAC is used to identify the transient stability type and to fast calculate the stability index. The advantages of this method is that it avoids machine group division， equivalence， and needs less information and little computation cost. Moreover， it can obtain the same accuracy as time domain simulation method. The validity of the proposed method is proven by an example system.