The mud return line is the key equipment of deepwater riserless drilling system. It is filled with high flow velocity mud， and the mechanical characteristics of mud on the line are not clear. The mechanical model of the mud return line under the combined action of internal and external flow was established by using the microelement method， considering the internal flow inertia force， friction force， centrifugal force and Coriolis force. The numerical model was established by using the finite element method， and the solver developed by Newmark-β method and MATLAB. The single effect and the combined effect of the internal flow force are analyzed， and the influence of mud flow rate， mud density and other factors on the internal flow mechanical characteristics is also studied. The result shows that the inertia force increases the vibration amplitude of the return line， riction decreases the lateral displacement of the return line， centrifugal force and Coriolis force increase the lateral displacement of the return line. The internal flow force has great influence on the return line， and the friction force has the greatest influence on the mud return line. The effects of inertia force， Coriolis force and centrifugal force are small. The effect of internal mud on the return line should be considered in the actual engineering analysis.