The broad-area laser diode is widely used as a pump source in all-solid-state lasers, while the beam of semiconductor laser diode have to be shaped because of large divergence angle and the asymmetry of the narrow rectangular cross section in fast-axis and slow-axis directions. The output laser spot of area-broad diode laser with bright area: 1 μm(fast axis)×200 μm(slow axis) was rectangular and the mode characteristics of multiple filaments in the direction parallel to the p-n junction was analyzed. The far-field optical intensity distributions was simulated based on ZEMAX ray tracing software that operated in a non-sequential mode by setting appropriate filament size, spacing and multi-longitudinal mode. The simulation result was consistent with the real optical intensity distributions. Then a compact optical system of beam shaping was presented which consisted of a cylindrical lens and self-focused lens. The cylindrical lens was responsible for collimating the beam along its fast-axis, while the self-focused lens made the spot size become smaller. Finally, the image size was 0.15 mm×0.17 mm and beam divergence angle was 3.3°×2.4° in fast-axis and slow-axis directions, respectively, at 1.8 mm after self-focused lens. In the end, the comparison between the simulated intensity distribution and the experimental intensity distribution was performed step by step after placing optical element. It is shown that the method of simulating rectangular spots based on the broad area laser diode is feasible by introducing the filament structure.