An experimental study is conducted using the single-factor experimental method on a closed-loop heat pump drying system incorporating air bypass and series-connected external auxiliary condenser. The research focused on analyzing the influence principles of material loading, circulating air velocity, expansion valve opening and air bypass rate on the specific moisture extraction rate (SMER). The results demonstrated that under optimal refrigerant charge (645 g) and identical drying duration, the system exhibited an optimal material loading of 1.845 kg. SMER is initially observed to increase then decrease with rising air circulation velocity. Superheat is increased and evaporation temperature is decreased by reducing the expansion valve opening, leading to enhanced dehumidification capacity and SMER. Maximum energy consumption reduction of 10.7% and SMER improvement of 20% are achieved by this configuration compared to 5 ℃ superheat conditions. An optimal air bypass ratio of 0.6 is identified for the system, resulting in a 35.7% SMER enhancement relative to non-bypass operation.