To solve the problem that the power density of laser beam attenuated along the propagation direction, which was caused by large beam divergence angle (14°-46°) of semiconductor laser diode, a beam shaping optical system to improve power density was proposed. An X type planoconvex cylindrical lens and a Y type planoconvex cylindrical lens were selected to collimate the laser beam. Then, a pair of planoconvex spherical lenses were applied to improve the parallelism. Finally, a single planoconvex spherical lens focused the laser beam to a spot with high power density. The lenses were chosen and optimized through Light Tools simulation. A real beam shaping optical system was mounted and built. Test results show that 67% laser power is converged into a cycle with diameter of 1 mm, and the power density is over 30 W/cm2.