The speed of a mechanical switch is a key factor to the performance of HVDC circuit breaker. The electromagnetic repulsion mechanism（ERM） has the characteristics of short motion delay and fast opening speed

which is suitable for the application of fast mechanical switch. However

the driving principle of eddy current repulsion force also leads to low energy conversion efficiency. In order to improve the power efficiency

3 D-finite-element simulation model of ERM is established

and the accuracy of simulation is verified by 40.5 kV/2 000 A mechanical switch prototypes. Then

by analyzing the distribution of magnetic field in the process of mechanism motion

it is pointed out that large magnetic flux leakage is the main cause of low efficiency. Finally

in order to reduce the space magnetic flux leakage and improve the power efficiency of ERM

based on the original hollow coil structure

the influence of adding different soft magnetic materials is analyzed. The simulation results show that the core material with low conductivity and high permeability

the radially slotted core

and the laminated core

can effectively optimize the magnetic circuit and improve the power efficiency of ERM.