Abstract: One of the conditions for sealing in three screw pump with cycloid profiles was that the length of case must not be less than the minimum working length, so that the suction and discharge chamber could be separated strictly. This study deduced the conditions from the equation of meshing that the length of case must meet, and gave the three dimensional graph of different sealing chamber. This study provided the theoretical basis for correct determination of case length, analysis of the instantaneous flow rate, pressure distribution and volumetric efficiency in three screw pump.