Voltage stability prewarning based on the PMU technology needs to monitor operation of power transmission lines in real-time. The existing branch voltage stability index only can be used in the case of the constant power factor because it ignores the effect of reactive power. A new index considering branch active and reactive comprehensively was proposed in this paper. The index is applicable to various load power factors. Detailed derivation was made on this index. The index was tested in IEEE14 bus system, and the results indicate that it could reflect branch operation and voltage stability conditions better.