The invention discloses a calculation method for a transient stability evaluation index of a system after a multi-loop DC commutation failure, and the method comprises the following steps: simplifyingsending and receiving grids into a double-cluster system, and extending the double-cluster system into a three-cluster system which comprises three regions; determining a power angle difference through a Runge-Kutta method during the removing of a fault of the receiving end grid of the three-cluster system, calculating the speed-up area and slow-down area according to the power angle difference and a maximum cutting angle, and determining the stability margin. The method also comprises the steps: analyzing the relation among an inertia time constant, a fault duration time and the stability margin, and obtaining the impact on the stability of a system from the inertia time constant and the fault duration time. According to the invention, the double-cluster system is extended into the three-cluster system, and the extreme cutting angle is determined through the Runge-Kutta method. The stability margin of a power system is calculated, and various demands of actual application are met.