In this paper, an InP-based mesa-structure uni-traveling-carrier photodetector is designed. By adopting Gaussian doping scheme in the absorption layer and incorporating an appropriate cliff layer, high speed and high saturation current characteristics are both achieved simultaneously. For the device with a 14 μm2 active area, the simulated results indicate that the bandwidth reaches 58 GHz and DC saturation current increases up to 158 mA at a reverse bias of 2 V. Under high optical injection, the bandwidth degradation and current saturation are studied, which are caused by energy band shift and electric field collapse.