Aiming at the flow characteristics of tubes, this work analyzed the different cases of inner flow in loop tubeby CFD, and the cases were determined according to the different radius of tube section. The CFD analysis results show that with the increase of radius, the highest velocity at mixing point decreases with a maximum value of 45.22%, but increase at the outlet with a maximum value of 4.79%. There are nonlinear change of flow velocity at the inlet and outlet, and the velocity in the tubes is layered vertically.