We design a double-semi-cylinder laser in order to solve the problem that the beams of slab laser, disk laser and tube laser are difficult to focus because they are not round and the problem of rod laser with a poor thermal effect. The beam of double-semi-cylinder laser can be round and its thermal effect is much better than rod laser. Results of numerical analysis with ANSYS software show that the highest temperature of double-semi-cylinder laser is about 25 ℃ lower than that of rod laser; its largest temperature difference is about 11 ℃ lower, and its largest thermal strain is less than 60% of that of rod laser.