To fully make use of the value of BeiDou signals big data resources and expand its applications in atmospheric and oceanic fields, a method to monitor rainfall information using polarimetric phase shift of BeiDou signals was studied theoretically. Based on the frequency and polarization characteristics of BeiDou signals, the microphysical process of BeiDou signals passing through rain area and interacting with raindrops was analyzed; from the definition of polarimetric phase shift, the mathematical model between polarimetric phase shift and rainfall intensity was established and analyzed by numerical simulation method. It is concluded that the polarimetric phase shift of BeiDou signals is sensitive to rainfall intensity, which illustrates the feasibility of monitoring rainfall intensity by BeiDou signals. The influence of some key factors such as raindrop size distribution, path length of rain area and elevation angle of satellite on polarimetric phase shift is systematically studied, and reasonable suggestions are put forward for subsequent experimental verification.