Extreme precipitation often causes enormous economic losses and severe disasters. Changes in extreme precipitation potentially have large impacts on the human society. In this study, we investigated the changes in four precipitation extreme indices over China during 1961 ~ 2014. The indices include total wet-day precipitation (PRCPTOT), precipitation on extremely wet days (R95pTOT), number of extremely wet days (R95d) and precipitation intensity on extremely wet days (R95int) during the extended summer (May-August). Observation analyses showed that these four indices have significantly increased over southeast China (SEC) and northwest China (NWC) whilst decreased over northeast China (NEC) and southwest China (SWC). Based on HadGEM3-GC3.1 historical, greenhouse gas only (GHG) and anthropogenic aerosol only (AA) simulations, we assessed the relative roles of different forcings in the observed trends. Model reproduced the main features of increasing trends over SEC and NWC in historical simulations, suggesting a dominant role of forced changes in the trends of four indices over the two regions. Individual forcing simulations indicated that GHG and AA forcings influence the increases in summer extreme precipitation over SEC and NWC, respectively, through different processes. Over SEC, extreme precipitation increase is mainly due to GHG forcing that results in moisture flux convergence increase through thermodynamic and dynamic effects. In comparison to GHG forcing, AA forcing has a weak contribution because AA forced moisture flux convergence increase is offset by AA forced evaporation reduction. Over NWC, extreme precipitation increase is primarily attributed to AA forcing and secondarily to GHG forcing. AA forcing can result in moisture flux convergence increase through dynamic effect, and GHG forcing can result in evaporation increase.