Aiming at the questions encountered in various state estimation algorithms for AC/DC combination system, a new state estimation algorithm is proposed in this paper, which can make AC active power, reactive power and DC equations decoupled fast. In the process of state estimation, AC and DC equations are solved in turn, keeping all high order derivatives of Taylor series for the DC equation. Thus, the Jacobian matrix related to the DC equations can be constant in the iterative process, which decreases the computation burden. On the other hand, it also makes the proposed algorithm have good convergence performance. Basing on the IEEE 14-bus system, a multi-terminal DC/AC combination system is established and simulated. The simulation results show that the algorithm is effective and feasible.