This paper presents a high step up SEPIC-based partial-power converter. The partial-power Flyback circuit and the modified SEPIC circuit are connected in cascade therefore the voltage gain of each cell is multiplied, obtaining high voltage gain without extremely high duty cycle over a wide input range. As for this two-stage converter, the first stage is the partial-power Flyback which is responsible for regulating the voltage within the wide input range and the second stage operates in a fixed state while providing zero-voltage soft switching. Compared with the full power converter, the partial-power Flyback converter processes a fraction of the power that flows from the source to the load, reducing losses and increasing power density. In this paper, the operational principle and parameter design process are introduced in detail. A 100 W prototype is designed to demonstrate theoretical analysis.