A transmitting-focusing device is designed for high energy laser(HEL) focusing in order to satisfy the demand of the engineering application of HEL system.Firstly,the design model of a catadioptric two-step beam expander system is selected,and then a design result of plane wave beam focusing in different distance is optimized through using optical design software in sequential mode.Secondly,the correction should be calculated by the theory of matrix optics because of the inconformity between plane wave beam and Gaussian beam in focusing adjustment quantity.Thirdly,the same optical system which uses incoherent combination laser in space as incident is refactored in non-sequential mode,and the simulation of HEL in transmitting and focusing is verified.This new designed optical system is used to contrast to an ordinary off-axis reflective optical system under the condition of same system parameters.The comparison proved that the new designed optical system which has better optical performance can satisfy HEL focusing in the range of 0.5~5km.Finally,a prototype experiment is carried out to verify the correctness and rationality of the design.