Abstract: The Northeast Cold Vortex (NECV) is an important weather system affecting Northeast China. In this study, based on multi-source observations and ERA5 reanalysis, an NECV rainstorm that occurred in Liaoning Province on June 9–10, 2021 (hereinafter referred to as the "21•6" NECV rainstorm) is selected to analyze the NECV activity and precipitation characteristics, and multi-scale dynamic and thermal diagnoses are conducted. Then, FLEXPART and the quantitative contribution analysis method of moisture sources are utilized to track the moisture sources, exploring the formation mechanism of the NECV rainstorm. The results are as follows. (1) During the NECV rainstorm, the intensity of NECV (north vortex) first increases and then decreases, with its radius expanding and then contracting accordingly. The average precipitation in Liaoning increases rapidly in the early stage and peaks from night to morning and from afternoon to evening. Heavy precipitation occurs on the windward side of the terrain. (2) The upper-level frontal zone is formed when the warm, humid air in front of the NECV trough interacts with the dry, cold air carried by the NECV. Liaoning is in a favorable position for the coupling of the high- and low-level jets, which is affected by the southerly weather pattern low-level jet in the southeast of the NECV. The convergence position of the wind at the top of the boundary-layer jet corresponds to strong precipitation. (3) In the early stage of precipitation, a convective cloud initially triggers at the southwestern coast of the Liaodong Peninsula, then moves eastward and develops under the conditions of convective instability, high temperatures and humidity near the ground, and southeast winds from the Yellow Sea and Bohai Sea convergence towards the shore. During heavy precipitation, convective clouds in the low-level jet that exhibit the "train effect" continuously move towards southern Liaoning, triggering strong upward movement. (4) The major moisture source regions contributing to the NECV rainstorm are east-central China and its coastal regions (35.69%), the western North Pacific region (33.69%), the South China Sea-South Asia Islands (14.72%), and the target region itself (6.38%). The moisture uptake of the first three regions is particularly significant. Despite a lot of moisture being released along the way, the final moisture contributions still remain high.