The shape of transition section has great influence on the flow pattern in open channel. The aim of this research is to address the problem of flow diffusion and the coherence between transition section and downstream flow, and to eliminate the impact of diamond-shaped shock wave in the downstream of open channel. According to the characteristics of shock wave and the basic relationship between shock wave and radical flow, we calculated the shape of the transition section with curved side-wall in open channel by adopting the simplified integral equation of shock wave following Liu Hansheng's formula of slit-bucket with curved side-wall. The flow coherence between the transition section and the downstream was also considered. This method can calculate the water surface curve in the transition section. Test results show that the flow in the transition section of open channel is smooth and stable, and water surface rises gradually. After the transition section, the water flow is normal, which is uniform flow, with no impact of diamond-shaped shock wave, demonstrating that this method could solve the problem of flow diffusion and coherence, hence can be applied to the design of transition section with curved side-wall.