Using the split-step Fourier method to solve the steady state traveling wave equation of tapered semiconductor laser amplifier, the optical and thermal characteristics of two kinds of laser amplifiers (with linear and nonlinear angle-opening structures) were analyzed by numerical simulation. By comparing the input current-output power curve, input power-output power curve and the number of filaments of the two laser amplifiers, the formation mechanism of filaments in the amplifiers was studied, and the reason of the different optical field distribution in the two laser amplifiers was explained. Results show that the nonlinear angle-opening structure can not only make the gain distribution more consistent with the optical field distribution, but also reduce the coupling between the reflected light and incident light at the edge of the waveguide. Furthermore, the tapered laser amplifier with nonlinear angle-opening structures has a higher optical-optical conversion efficiency and a more stable mode output.