The drift-diffusion theory is adopted to simulate the bandwidth and the saturation characteristics of InGaAs/InP uni-traveling-carrier (UTC) photodetector. According to the experiment results, we analyze the physical mechanisms in high speed and high power UTC photodetector. It is shown that introducing the cliff layer and the gradually doped absorption layer can enhance the saturation characteristic obviously. Electric field collapse in the absorption layer leads to the saturation, and RC constant is still a limitation factor for device with diameter larger than 20 μm. In this paper we point out the effective methods to improve device performance.