An apparatus and a method for detecting refrigerant leakage in an air source heat pump system. The method for detecting refrigerant leakage in an air source heat pump system includes the following steps in a cooling mode: S110: obtaining a running parameter of an air source heat pump system, wherein the running parameter at least includes a compressor rotational speed; S120: comparing the running parameter with a preset running parameter range; S130: updating a cumulative score when the running parameter falls within the preset running parameter range; and S140: when the cumulative score exceeds a predetermined cumulative score, determining that refrigerant leakage occurs, and when the cumulative score does not exceed the predetermined cumulative score, return to step S110.