To clarify the occurrence patterns of short-duration heavy rainfall (SDHR) before and after monsoon onset and to improve its intensity classification forecasting during the first rainy season in Huizhou, this study uses the hourly rainfall data from 201 automatic weather 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 spatial-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. The results are as follows. (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 northern area and the southern coastal area. The highest mean rainfall is observed in the southern coastal area in both periods. However, the extreme characteristics of the maximum hourly rainfall are more pronounced in the northern area and the southern coastal area during the summer monsoon rainfall period. (2) In terms of diurnal variation, during the frontal rainfall period, SDHR is frequent during midnight-early morning and late morning-afternoon, with the most intense rainfall occurring in the afternoon. During the summer monsoon rainfall period, SDHR is concentrated during early morning-late morning and afternoon-evening, with the highest intensity shifting to the early morning. (3) Most SDHR lasts 1−2 h, with a relatively higher frequency of 2−3 h 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 higher than ordinary SDHR. During the summer monsoon rainfall period, q₈₅₀ remains consistently high with large overlap between two types of SDHR, 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 first rainy season in Huizhou.