A transformerless high step-up DC-DC converter is investigated in this paper. Two buck-boost converters are integrated with single switch. The input voltage of the front semi-stage is the DC source. The input voltage of the rear semi-stage is the series of the DC source and the output voltage of the front semi-stage. Moreover, the output voltage of the proposed converter is the output voltages of the two semi-stage by cascode. Thus, high step-up voltage gain can be achieved with appropriate duty ratio. The operating principle and steady-state analysis of the proposed converter are discussed in detailed. Finally, a laboratory prototype circuit is implemented to verify the performance.