The invention discloses a car frame longitudinal member welding tool fixture which comprises a main base. A moving device is arranged on the main base; a clamping device is arranged at the upper end of the moving device; the moving device comprises slide rails; a horizontal sliding plate is arranged on the slide rails; a horizonal hydraulic cylinder is arranged on the main base and is connected with the horizontal sliding plate through a connecting rod; and the clamping device comprises a first clamping device and a second clamping device. The car frame longitudinal member welding tool fixtureprovided by the invention is applicable to various car frame longitudinal members with different lengths, and a position of the horizontal sliding plate on the main base can be adjusted according tothe length of the longitudinal member, so that the tool fixture is applicable to the car frame longitudinal members with different lengths; and when the car frame longitudinal member welding tool fixture provided by the invention is used for clamping the car frame longitudinal members, one car frame longitudinal member is conveyed to an assigned position through an overhead crane and is placed onto the fixture, and the car frame longitudinal member can be instantly clamped through fast actions of a left clamping assembly and a right clamping assembly, so that the aim of improving the working efficiency is achieved.