Abstract: An extremely warm-sector severe torrential rain event occurred in Xiamen on 18 April, 2023, characterized by high intensity, strong locality and hard to predict. The synoptic background, mesoscale characteristics and the role of low-level jets were analyzed using multi-source detection data such as conventional observation data, automatic meteorological stations data, dual polarization Doppler radar, wind-profile radar and disdrometer. The results are as follows. (1) The southerly synoptic-system-related low-level jet (SLLJ) and southeasterly boundary layer jet (BLJ) are the main influence systems, whose significant development improved local water vapor and instability conditions. (2) The linear mesoscale convective system (MCS) embedded with quasi-supercell storm, formed and developed in the left side of the low-level jet. It has been maintained for a long time along the southern coast of Fujian, exhibiting low echo centroids and high precipitation efficiency. This combination causes severe torrential rain due to the"train effect". (3) The ZDR, KDP, CC are all large in the strong echo area of the quasi-supercell storm, with a lower centroid height compared to the classical supercell storm. High concentrations of small and large raindrops are the main cloud microphysical characteristics of warm-sector torrential rain. (4) The rapid intensification and downward expansion of the low-level jet is closely related to heavy rain, and the ascending motion produced by the coupling of the double low-level jets was the main synoptic-scale triggering mechanism for this warm-sector heavy severe torrential rain.