The dynamic process of power system after DC blocking will last for a long time. The boiler dynamics should be taken into consideration when calculating the system frequency response. Firstly, the low order frequency response model of the DC receiving-end system is derived. The boiler and the coordinated control system of the thermal power plant are introduced briefly. Then, the non-mechanism modeling method is adopted to establish the transfer function model which describes the mechanical power-frequency characteristics of the generator. The load disturbance test is carried out and the frequency and mechanical power deviations of the generator are measured. The power spectrum and the least square methods are applied to identify parameters of the transfer function model. The selection of load disturbance signal and samples is discussed. Finally, the proposed method is applied in the IEEE 3-generator 9-bus system with a DC feed line. The long-term frequency response of the receiving-end system after DC blocking is calculated. The results validate the effectiveness of the identified model considering boiler dynamics. This work is supported by National Natural Science Foundation of China (No. 51837004) and Science and Technology Project of State Grid Corporation of China “Research and Application of Large-scale Pre-dispatch Key Technologies” (No. SGZJ0000KXJS1800285).