The eastward propagation of low-pressure systems from the Tibetan Plateau (TP) is one of the most important factors that contribute to the extreme Meiyu rainfall over the Yangtze River valley in 2020. This study investigates the diurnal variations of low-pressure systems over the TP during the Meiyu period and the possible mechanisms involved. The activity of low-pressure systems is characterized by a prominent diurnal cycle and its formation is closely related to the thermal contrast between the near-surface and the lower atmosphere. In the morning, diabatic heating near the surface increases sharply after sunrise, due to the increase in surface sensible heating, leading to a decrease in diabatic heating with increasing height. As a result, PV weakens in the lower atmosphere and the formation of low-pressure systems reaches a minimum. However, as turbulence increases, evaporation near the surface strengthens and reduces the surface diabatic heating. At the same time, the evaporated water vapor contributes to the formation of clouds at around 400 hPa, where diabatic heating increases due to the associated condensational latent heat release. Consequently, the thermal contrast below 400 hPa changes to the increase in diabatic heating with height, which favors the increase in PV and the generation of low-pressure systems. The timing of the arrival of low-pressure systems at the eastern flank of the TP plays an important role in the subsequent development of the systems. Early arrival is generally accompanied by air convergence and sufficient water vapor supply in the mid-troposphere downstream of the TP. This favors the systems moving off the TP and then influencing precipitation over the Yangtze River Basin.