Abstract: In order to overcome the problems encountered in the process of deepwater drilling,the AGR Subsea Company proposed the Riserless Mud Recovery(RMR) drilling system.Ocean riser is eliminated in this system,mud and cuttings are pumped to drill platform with minor diameter return line.Because RMR is a new technology and it needs correct hydraulic computational theory and method urgently. On the basis of the operating condition and characters of RMR system, developed hydraulic computational formula of RMR system and compiled hydraulic software, gave an example and compared with the data of literature, verified the correctness of the calculation formulas and procedures.