Abstract: Using the hourly precipitation data of 80 national stations in Heilongjiang Province, the NCEP/ NCAR and the EC-ERA5 reanalysis data, the characteristics of heavy precipitation and low-level jet during the Northeast Cold Vortex (NECV) rainstorm process and the diurnal variation of heavy precipitation in the warm season (from May to September) from 1981 to 2022 are analyzed. Several typical cases at four times a day are synthesized to analyze the causes of diurnal variation of heavy precipitation frequency. The results show that: 1) In the warm season, more than 80% of NECV rainstorm cases are accompanied by low-level jet. The 850 hPa jet stream at 850 hPa has the highest frequency and the longest duration, which is indicative for the prediction of heavy precipitation and precipitation extremes. 2) The southerly low-level jet is the strongest in the afternoon, and the southeast low-level jet is the strongest in other periods. Both jet streams have significant vertical wind shear at 850 hPa. 3) The peak of the diurnal variation of heavy precipitation frequency appear in the afternoon, and the trough of the diurnal variation appear in the late night. The highest frequency region of heavy precipitation is the southeast quadrant of NECV, followed by the northeast quadrant. 4) The distribution of precipitation at night is uneven, the spatial gradient is large, the heavy precipitation is related to the activities of the dry cold air at the middle level, and the convective instability is more conducive to the occurrence of heavy precipitation. During afternoon to early midnight, heavy precipitation occurs in the area with the largest horizontal wind edge on the left side of the low-level jet stream. Small-scale vertical circulation caused by complex terrain can enhance the precipitation.