In this paper, a design for a high extinction ratio Mach-Zehnder optical modulator is proposed. The switchable refractive index of the germanium-antimony-selenium-tellurium (GSST) phase change material is employed to induce destructive interference between the waves passing through Mach-Zehnder interferometer (MZI) arms and to realize amplitude modulation. A novel, to the best of our knowledge, asymmetric input splitter is designed for the MZI to compensate for unwanted amplitude differences between MZI arms and increase the modulator performance. Three-dimensional finite-difference-time-domain simulations show a very high extinction ratio (ER) and low insertion loss (IL) of 45 and 2 dB, respectively, for the designed modulator at the wavelength of 1550 nm. Moreover, the ER is above 22 dB, and the IL is below 3.5 dB in the wavelength range of 1500-1600 nm. The thermal excitation process of GSST is also simulated using the finite-element method, and the speed and energy consumption of the modulator are estimated.