A combined photovoltaic DC-DC boost circuit is improved according to the step-up ratio of traditional boost circuit is not high. Boost and half-bridge circuit is adopted in series combination to achieve boost inverter circuit in front-end of circuit structure. Isolation transformer can realize two step voltage transformations. An improved three-level configuration is adopted in the secondary side of secondary side phase-shift control. The numbers of IGBT are increased with an improved circuit. All of the IGBT in circuit achieved soft switching and reduced loss during switch process. In addition, the efficiency of circuit is improved. This circuit has the following characteristics: 1) The structure of circuit is simple and clear; 2) The gain range of average voltage is higher and provides a high voltage for back-end of inverter; 3) The soft switch can be achieved in all IGBT to enhance efficiency.