•A reduced-order equivalent thermal parameters (ETP) model is adopted.•The model is simplified into a linearized form.•Free from uncertainty of the indoor mass temperature.•Free from being influenced by inconsistent sampling intervals. Air conditioners (ACs) are considered to be one of the best candidates in demand response (DR) programs due to their heat/cold storage capacities. The potential of ACs in DR is to a great extent dependent on their thermal parameters. Existing ACs’ parameters estimation methods have difficulties in dealing with varying sampling intervals and unmeasurable mass temperature. To fill this gap, this paper proposes an ACs’ thermal parameters estimation method based on reduced-order equivalent thermal parameters (ETP) model, which is applicable of data with inconsistent sampling intervals and unknown mass temperature. After the model is converted into a linearized form, the parameters can be derived through least square (LS) or recursive least square (RLS) method. Testing results based on real ACs data verify the effectiveness of the proposed method.