The invention provides a noiseless fan structure which comprises a case, an air inlet, an air outlet, a volute, a wind wheel and a motor, wherein the size of the air outlet is less than or equal to that of the air inlet; the volute comprises an inlet base circle, a volute profile, a diffuser and a volute tongue; the diffuser is expanded along the rotating direction of the volute; the expansion angle a of the diffuser is 6 to 8 degrees; the diameter of the inlet base circle is D, and the radius r of the volute tongue is (0.025-0.05)D; a distance k between the volute tongue and the inlet base circle is (0.04-0.1)D. The air outlet speed is greater than or equal to the air inlet speed, so that an enough amount of incoming air can be supplied to the wind wheel, and noise generated by vacuum is avoided; the expansion angle of the diffuser can reduce the air flow speed of the air outlet and convert part of kinetic energy into a static pressure value; meanwhile, the frequency of air flow entering the inlet of the diffuser from the wind wheel is different from that of air flow entering the air outlet from the diffuser, so that resonance caused by the air flow in the diffuser is avoided, and no noise is generated; when r and k are valued in the range, the fan has lower noise and is higher in efficiency.