The path asymmetrical and refractive index asymmetrical design for optical waveguide methods have been introduced which can make the Mach-Zehnder modulator to achieve optical bias.The simulation studies of the two kinds of optical bias methods are based on finite difference-beam propagation method (FD-BPM).The results show that both the two methods can make the Mach-Zehnder modulators to be biased at half-power point in the most linear region.The initial phase difference of the two arms can reach around π/2,and the extinction ratios are 26 dB and 23 dB respectively.