Abstract: The AC filter bank and reactive power compensation device of the line commutated converter（LCC） occupy a large area, and LCC is prone to commutation failure when the AC power grid fails. The evolutional shunt capacitor commutated converter（ESCCC） can reduce the occupied area of the converter and improve the ability to resist commutation failure, but the realization of unity power factor control through the valve-side capacitor switching will affect the harmonic characteristics. To solve these problems, and considering the optimization of DC line loss, this paper proposes a new unity power factor control strategy based on ESCCC combined with the capacitors that can be switched on the AC bus. Firstly, the topological structure and mathematical model are established, and the principle of unity power factor control is analyzed. Secondly, a new control strategy is proposed, and the filter inductance, capacitance and other parameters are designed by adopting the new harmonic evaluation index.Finally, a simulation model is built in PSCAD/EMTDC to verify the effectiveness of the control strategy and the rationality of the parameter design. The simulation results show that ESCCC can achieve unity power factor control under all operation conditions,and reduce the DC line loss and the probability of commutation failure.