This paper proposed a step-down partial-power optimizer (PPO) structure to realize the Maximum Power Point Tracking (MPPT) for PV series-connected optimizer system. In the traditional power optimizer, the whole amount of PV panel power is processed by the converter, whereas in a partial-power optimizer only a fraction of the full panel power is processed. Therefore, the size and cost of the power optimizer are decreased. The proposed PPO structure is a buck structure, which mainly consists of an isolated DC/DC converter. Comparing with the boost structure, the buck structure can allow more PV panels and more importantly, the buck converter can operate normally even when bad partial shading occurs. A simulation and a hardware platform are built to verify the proposed PPO structure. The result shows good tracking accuracy and high efficiency of MPPT.