During the period of 28 June to 1 July, 2016, a Tibetan Plateau(TP) Vortex was generated, which then developed and moved eastward to subtropical region of China, resulting in precipitation in the lower reaches of the Yangtze River. The potential vorticity (PV) is used to diagnose the process by using the second Modern-Era Retrospective analysis for Research and Applications (MERRA-2) data and the Tropical Rainfall Measuring Mission (TRMM) precipitation data. Results indicate that surface heating over the TP pocesses obvious diurnal variation. It changes from heat source in the daytime to cold source in the night and directly influences the vertical gradient of diabatic heating. Negative PV is generated near the surface in daytime and positive PV is generated in night, demonstrating a prominent diurnal cycle. When the night-time positive PV generation becomes very strong and cannot be compensated for by the daytime negative PV generation, the TP low vortex is formed. By the time the low vortex system moves to the eastern TP, diabatic heating associated with strong precipitation reinforces the vortex. As the low vortex system continues to propagate eastward, the PV advection increasing with height acts as the large circulation background over the middle and lower reaches of the Yangtze River, which is in fever of the development of air ascent, resulting in occurrence of precipitation.