To improve the precision of power system dynamic equivalents， the system being studied is classified through the critical-modes and participation factors based on extended equal area criterion（EEAC）. First of all， all generators of the critical cluster are retained， and so do the generators in the remaining group which have a larger absolute value of participation factor; then， the nodes are retained as the buffer nodes whose electric distances are closely connected to the above mentioned generators; finally， the coherency-based equivalence is used in the rest of the system. In addition， an index is also proposed to evaluate the accuracy of the dynamic equivalent. Simulation results show that by use of above-mentioned rules， the important details of dynamic model for external subsystem can be reserved.