Wenchuan Earthquake triggered a large number of geological hazards, dramatically stimulating soil erosion. This study was carried out in Pengzhou County, Sichuan Province. By comparison of sediment, runoff and nutrient losses in earthquake-damaged forests (EF) and unaffected forests (UF), the actual status of soil erosion after the Wenchuan Earthquake was investigated by runoff plots. Results showed that water and soil losses were dramatically increased after earthquake. During the study period (from August to November 2010), UF runoffs were 19.26, 36.76, 10.68 and 7.51 L m−2, while total runoffs in EF sites were 30.41, 25.79, 5.03 and 2.67 L m−2 respectively, which were 15, 15, 18 and 19 times more than those in UF. Total sediment losses in EF sites were 28.94, 25.16, 4.11 and 1.98 t km−2 respectively while in UF they were 707.69, 610.05, 113.43 and 58.95 t km−2 respectively during the same study period, i.e. 23, 23, 32 and 29 times more than those in UF. Path analysis showed that both vegetation and rainfall exerted an indirect influence on sediment loss by significantly influencing runoff, which correlated with sediment loss very significantly. Although no obvious differences of the nutrients’ concentration in runoff water (soluble organic carbon (SOC), total nitrogen (TN), total phosphorus (TP) and total potassium (TK)) between EF and UF sites were observed, total losses of the four nutrients were significantly higher in EF than in UF sites (for example, in EF sites, SOC, TN, TP and TK losses were 970.52, 114.46, 2.26 and 307.00 g m−2 respectively, while in UF they were 38.13, 4.22, 0.10 and 13.28 g m−2) due to significantly higher runoff in EF sites. In conclusion, soil erosion was significantly more serious due to the loss of forested lands resulting from the Wenchuan Earthquake, delaying the restoring process of forest cover and weakening the ecological linkage between upstream and downstream.