On the night of June 9th and June 12th, 2021, Shaoxing, Zhejiang Province, experienced two warm-sector heavy rainfall. Both subjective and objective forecasts show large deviations. Using the ground-based automatic station, Doppler radar observation data and ERA5 reanalysis data, the circulation field, triggering mechanism and mesoscale convective system evolution of the two processes are analyzed. The results are as follows: (1) Both processes occurred under the background of weak weather forcing. The “609” process took place under the action of boundary layer jets. While the “612” process occur red at the edge of the subtropical high. (2) During the “609” process, the centre of the large vertical helicity stretched up to the middle troposphere, and the upstream potential vorticity disturbance was continuous transportation, promoting the continuous development of the potential vortex disturbance in the rainstorm area. The large vertical helicity centre of the “612” process only extended to the lower troposphere, and there was dry and cold air intrusion on the isentropic surface, which was conducive to the rapid strengthening of the potential vortex disturbance. (3) The triggering mechanisms of the two processes were both β mesoscale shear lines. The mesoscale convective system developed continuously along the shear line and was in a “quasi-static” state, causing heavy rainstorms. The β mesoscale shear line and the mesoscale convective system of the “609” process had a longer stagnation time than the “612” process and were affected by the windward slope’s uplift, so the total amount of precipitation was greater.