Power system reliability，which acts as a reference of power system planning and operation，will be influenced by the integration of energy storage system（ESS）.Optimizing planning and operation schedules of ESS with reliability consideration is a topic worth study.However，such optimization is restricted by the low efficiency of existing reliability assessment algorithms after ESS is integrated.For this reason，improvement on the sampling algorithm is made to enable it to estimate the reliability impact caused by ESS accurately and efficiently while providing the possibility of solving the problem of optimization restricted by reliability.Then，a comparison is made of the algorithm in question with the time sequential Monte Carlo simulation method in an IEEE RTS79 test system.Calculation results show that the improved pseudo-sequential state transition sampling algorithm is capable of maintaining good accuracy of errors（relative error being less than 3%），while boosting the single-thread computation efficiency ten times or thereabouts and with even higher computation efficiency in a multi-thread environment.