Aiming at the engineering application demand of current high energy laser system, a transmission high power laser emitting system with continuous focusing and focusing is designed in the range of some miles from the target. Firstly, the far-field focusing characteristics of Gaussian beam are analyzed theoretically, and the parameters such as amplification ratio, and angle of divergence are calculated. Then three, four and five pieces are selected as the design models, and the simulation analysis is carried out by using the optical design software sequence mode, and the design results of the three structures are analyzed and compared. In order to improve the stability and transmittance of the system, and to take into account the correction of aberration and the axial distance of the lens and other factors, the five-piece transmission type is selected as the structural form. The focusing curve of this structure is smooth and easy to be processed. It is composed of 3 fixed lenses and 2 focusing lenses, and spherical lens is used to realize the continuous focusing and focusing within the target distance of 0.3~2 km. The results show that the beam expansion ratio of the system is 6.5, the angle of divergence is about 63 μrad. The wavefront is better than 0.42λ in the whole focusing stroke, and the probability of the wavefront being better than 0.3 is 90% after the tolerance allocation. The design of the tolerance allocation is reasonable, and the structure is stable and reliable. As a result, the optical performance is excellent, and it has a good application prospect in the field of high-energy laser.