The influence of a multi-mesa structure on a pin photodiode (pin-PD) was carefully studied. It was found that PDs with multi-mesa structures form an uneven electric field in the device under the working voltage, and the electron obtains the overshoot velocity in the uneven electric field. The device that adopts the multi-mesa structure can also reduce the capacitance. The introduction of multi-mesa structures provides another way to improve the performance of the pin-PD. Compared with the one-mesa structure, the multi-mesa structure improved the bandwidth of 5.5 GHz and reduced the capacitance from 26 to 19 fF. The modified pin-PD with a multi-mesa structure can be used in 100-Gbits optical communication systems.