Abstract: In terms of the stability of the background field, a 2D Boussinesq approximation model is adopted, and from the prognostic field exported by the model on an hourly basis a background field during the rainstorm is taken, followed by the method for minimum of the function to get a stratification parameter and heating rate, with which to discuss the role of unstable disturbance during the rainfall. Evidence suggests that heat released from condensation exerts distinct effects on the generation and development of rainstorm instability. At the early and prime stages the environmental exhibits dynamically instable disturbance that propagates at a small angle with flow, i. e., instability of 2D plane wave. The torrential rain occurs under the joint action of the thermal and dynamic factors.