Provided is a fixture for automatic assembly, overturning and welding of sidewall aluminum profile of a rail vehicle, comprising a lifting mechanism, a supporting overturning device mounted on the lifting mechanism, an automatic assembling sidewall profile device mounted on the supporting overturning device, and a self-positioning locking device. The automatic assembling sidewall profile device comprises a sidewall-shaped support steel beam, and a rodless cylinder track platform and a fixing seat. The self-positioning locking devices are mounted on the rodless cylinder track platform and the fixing seat, and have a self-positioning base and a locking device. The self-positioning base comprises an outer housing is provided with the locking device and two symmetrically set self-positioning supports, and faces of the two self-positioning supports matching with the rail vehicle aluminum profile are provided with rollers having a V-shaped gap formed therebetween.