An inflating pump with auxiliary knotting functions for filling up air balloons has a cylinder, piston, actuating shaft, operating handle and outlet nozzle. The piston is set slidably into the actuating space of the cylinder. The inner end of the actuating shaft is linked to the piston, and the outer end protruded from one end of the cylinder. The operating handle is linked to the outer end of the actuating shaft. The outlet nozzle is set to connect with the actuating space of the cylinder. An auxiliary knotting portion is set with a spacing to the outlet nozzle and has a bulge and an internal flange formed at end of the bulge for knotting purposes. After the air balloon is filled up with air, the user could rewind the opening of the air balloon for auxiliary knotting by taking the bulge and internal flange as the guiding structure.