The non-coherence of the disturbed power system may lead to inaccurate dynamic equivalence and affect the analysis of power system safety and stability. For this reason, a quantitative analysis method for power system coherence considering the influence of damping torque is proposed. Firstly, based on the single pendulum equation, it gives a derivation of the dynamic equivalent system period characteristic expression after the power system is disturbed, and the correlation between the dynamic equivalence and the coherence of the power system is discussed. Secondly, in the scenario of determined faults, a motion period estimation method is put forward based on the upper and lower boundaries in the rotor trajectory of the equivalent system. A quantitative index is proposed by comparing the estimated and the measured periods. Then, the influence of damping torque is deeply studied, and the quantitative evaluation index of system dynamic damping characteristics is proposed based on the estimation period. Finally, the simulation examples of two typical systems show that the methods and indicators proposed are effective and reliable, which provide a reference for evaluating the safety and stability of the power system.