To clarify the occurrence patterns of short-duration heavy rainfall (SDHR) before and after the monsoon onset and to improve its intensity classification forecasting during the pre-flood season in Huizhou, this study uses hourly rainfall data from 201 automatic meteorological stations in Huizhou, covering the period from April to June over the recent 13 years (2012—2024), along with ERA5 reanalysis data. It systematically analyzes the spatio-temporal distribution of SDHR during two periods: the frontal rainfall period before monsoon onset and summer monsoon rainfall period after monsoon onset. Additionally, the precursor characteristics and favorable combinations of key environmental parameters for different SDHR intensities are investigated, leading to the preliminary establishment of reference indicators for intensity classification. Results show that: (1) Spatially, during the frontal rainfall period, SDHR occurs most frequently in the northern mountainous area, whereas during the summer monsoon rainfall period, it shows dual high-frequency centers in both the north and the southern coastal area. The highest average rainfall is observed in the southern coastal area in both periods. However, the most extreme hourly rainfall intensities occur in both the northern and southern coastal areas during the summer monsoon rainfall period. (2) In terms of diurnal variation, during the frontal rainfall period, SDHR peaks from midnight to early morning and from late morning to afternoon, with the most intense rainfall occurring in the afternoon. In the summer monsoon rainfall period, SDHR is concentrated from early morning to late morning and from afternoon to evening, with the highest intensity shifting to the early morning. (3) Most SDHR lasts 1—2 hours, with a relatively higher frequency of 2—3 hours during the summer monsoon rainfall period. (4) Convective available potential energy (CAPE) and specific humidity at 850 hPa (q850) are indicative for intensity classification. During the frontal rainfall period, extreme SDHR has higher CAPE and q850 than ordinary SDHR, with the optimal CAPE about 1.3 times that of ordinary SDHR. In the summer monsoon rainfall period, q850 remains consistently high with large overlap between intensity classes, while the optimal CAPE for extreme SDHR is approximately twice that of ordinary SDHR. This study preliminarily establishes intensity classification indicators for SDHR in different periods, providing a reference for refined SDHR forecasting during the pre-flood season in Huizhou.