With the ever-increasing DC transmission lines，research on cascading failures of the AC-DC system is becoming very urgent.A cascading failure model for the AC-DC system based on the improved OPA model is proposed.With dispatching and the main protection devices such as 100 Hz harmonic protection devices taken into account，it is possible to analyze the failure propagation process in the AC-DC system and the impact of DC system controllability on blackout risk.This model uses equivalent admittance parameter of DC system to calculate the short-circuit voltage，thus avoiding detailed time-domain simulation and significantly improving computational efficiency.Simulations are performed on a revised IEEE 30-bus system and Henan power grid.The results show that blackout risk of DC and AC-DC interconnected systems are higher than that of AC interconnected system with no long distance power transmission.Because of the good regulating ability of the DC system，the blackout risk of DC interconnected system can be appreciably reduced by appropriate impact of the dispatching.