The utility model provides a can reduce and draw compound molecule pump that backflows in grade clearance is provided with the gas -tight body structure of backflowing pulling between rotor and the stator, its includes hoop boss and hoop recess, hoop boss and hoop recess are each other just to setting up, and the axial direction line of hoop boss and hoop recess is coincident mutually with driving spindle's axial direction line, when pulling the rotor and set up the helicla flute in for a light section of thick bamboo, stator, the setting of hoop boss is on pulling the rotor, and the hoop recess sets up on the stator, when the helicla flute was offered for a light section of thick bamboo, on drawing the rotor to the stator, the hoop boss set up on the stator, and the setting of hoop recess is on drawing the rotor, the gas -tight body backflow the structure setting between viscous flow state department, molecule flow state and the viscous flow state of compound molecule pump transition flow state department or above -mentioned two kinds flow states department settings simultaneously, on axial direction, the width of hoop boss is less than the width of hoop recess, the interval is for pulling 1~5 times of between rotor and stator interval between hoop boss and the hoop recess.