This research investigates the mechanisms behind the development of a squall line that impacted the Hebei-Shandong area on 28 June 2022, as well as the factors leading to extreme short-duration heavy precipitation. By integrating multi-source observations with ERA5 reanalysis, it is observed that most numerical models fail to predict the rapidly developed and highly organized event, highlighting the difficulties in predicting such convective systems under weak low-level synoptic forcing. The study examines the initiation, organization and underlying mechanisms of the system, aiming to enhance the predictive capacity and disaster prevention and mitigation strategies for similar severe weather events. Key findings are as follows. (1) The convection occurs under weak low-level dynamical forcing (absence of both low-level shear line and cold front). However, the cold, dry air aloft and the warm, moist air near the surface lead to a highly unstable stratification and baroclinicity. Combined with high convective available potential energy, surface convergence lines are the primary trigger for convection. (2) The squall line’s organizational development involves the “embedded” merging of northern and southern convective cells. The RKW theory successfully explains its maintenance, and when the ratio of cold pool propagation speed to low-level vertical wind shear varies between 0.9 and 1.7, it is the most beneficial for the maintenance. (3) A mesoscale convergence system with significant intensity (divergence at most -11×10^-5 s^-1) and long duration (at least 1 h) is a reliable indicator for extreme short-duration heavy precipitation events. (4) During the extreme precipitation, the dual-polarization radar parameters display the typical characteristics of high Z_DR (＞2.0 dB), high K_DP[＞4.0 (°)·km^-1] and high C_c(＞0.98), suggesting that the precipitation is mainly composed of large, concentrated raindrops.