Using the method REI to simplify the large-scale power system, the equivalented resistance of electric trassimision line is maybe negative; such things don’t tally with the reality. The cause of this problem is that the nodes in external systems aren’t distinguished in the process of equivalence, so that the key nodes which make a big difference with the researched system’s stationary and transient characteristics, and have strong correlation with the researched system are eliminated. Aiming at this problem the node sensitivity analysis approach is proposed; according to the network power correction equations to define the system’s reactive power and active power sensitivity matrix, and combining the practical measurement sensitivity of the grid, the method to estimate the key nodes in external systems which must be reserved is put forward; and this process is introduced in coherent-based equivalence of large-scale power system. Using BPA simulation software and taking the New England standard 39-node system as the test example, the external system is simplified by the improved coherent-based equivalence method; the validity and the feasibility of this method is verified by the stationary and transient characteristics of the equivalented system.