The connection operation window of the mud return line can determine the environmental limits of safe operation, which is very important to ensure the safety of the riserless drilling system. To analyze the safe operation window of the mud return line, a mechanical model of the mud return line is established based on the infinitesimal method. The safe operation criteria of the mud return line under connected condition is proposed. The operation window analysis process of the mud return line based on the dichotomous method is established. According to the analysis of the operation window for the riserless mud return line, it is found that the safety of the mud return line decreases with the increase of current velocity and wave amplitude. The operation window of the mud return line is sensitive to the change in wave amplitude. The riserless drilling system is recommended when the current velocity is less than 0.8 m/s and the wave amplitude is less than 5 m.