The characteristics of flow regime transition in the power generation channel of a liquid metal MHD system is numerically investigated in the present paper, under the flow regime of stratified flow and annular flow at the inlet, respectively, based on a cuboid channel model. The findings indicate that, after the stratified flow and annular flow pass through the power generation channel, jet flows will be generated in the region of high gas volume fraction, and the maximum jet flow velocity will increase with the external magnetic field. Despite the different inlet flow regimes, the flow regime in the power generation channel transforms towards a mixed flow. In addition, a power generation channel with a liquid metal inlet at the top was designed, which can effectively transform the stratified flow into the mixed flow, and the output power of the generation channel grows with the accelerated liquid metal at the inlet.