The sensing mechanism of a dynamic thermal source pendulum single-axis MEMS thermal accelerometer was revealed in this paper. Based on the principle of the sensitive structure, the temperature field inside the sensitive structure is calculated by establishing the two-dimensional physical research model, meshing the grid, loading the acceleration and other methods. The results show that a stable temperature field centered at the dynamic heat source is formed within the sensitive structure after 1.8 s of power-on. When inputting the acceleration a, the dynamic heat source shifts along the direction of acceleration, then the temperature field is shifted. The temperature difference ΔTX between the two hot lines symmetrically set in the direction of sensitive axis increases linearly with the increase of input acceleration a, and the temperature sensitivity is 7.1×10-2 mK/g. According to the input-output characteristic a-VXOUT curve, the mathematical model is given, and the sensitivity of accelerometer is 0.5 V/g, and the nonlinearity is 2.8%, which reveals the sensing mechanism.