Abstract: On April 21, 2025, two widespread thunderstorm wind (Derecho) events with an interval of approximately 4 hours occurred sequentially along a corridor extending from northern Hunan through northern Jiangxi to Zhejiang. Both derechos intensified notably over Yichun, Jiangxi, where record-breaking Grade 12 gales were observed at four stations, resulting in severe damage. To improve understanding of the formation mechanisms of Derecho events and extreme gales, this study compares the characteristics, causes and intensification mechanisms of extreme winds in Yichun using upper-air and surface observations, dual-polarization radar data and ERA5 reanalysis data. The results are as follows. (1) The extreme winds in Yichun were produced by two Derecho events. Favorable environmental conditions for Derecho events occurrence include low-level warm advection, mid-level dry layers, deep 0–8 km vertical wind shear of 30.7 m·s⁻¹, and a mean entrainment-layer wind speed of 19.2 m·s⁻¹. (2) From 15:30 to 15:50 BT, the first Grade 12 gale in Yichun was jointly induced by multiple successive downbursts and density currents associated with cold pools. The cold pools produced a 1-hour temperature decrease of 10 °C following the merger and intensification of supercells and small convective cells. (3) The second Grade 12 gale in Yichun during 19:00–19:30 BT was generated by combined effects of downbursts and storm-scale momentum downward transport. Such momentum transport was characterized by a temporally continuous high-wind core of 27 m·s⁻¹ that rapidly descended from an altitude of 4–7 km to the ground. The findings provide some indicative implications for the nowcasting of extreme winds over Jiangxi in spring. When downbursts, momentum transport and strong cold pools occur simultaneously or persistently affect a station, rapid intensification of local winds should be alerted. Extreme winds can also develop in multi-cell storms via prominent downward momentum transport under environments with large entrainment-layer wind speeds.