The invention relates to the field of automobile frames, in particular to a tractor frame assembly with variable-section longitudinal beams and a forming process of the variable-section longitudinal beams. The tractor frame assembly with the variable-section longitudinal beams comprises a frame body composed of the two variable-section longitudinal beams, a first cross beam, a second cross beam and a tail cross beam, and a rear axle transversely arranged below the rear part of the variable-section longitudinal beams, wherein the first cross beam, the second cross beam and the tail cross beam are sequentially arranged between the two variable-section longitudinal beams from front to back, the cross sections of the variable-section longitudinal beams are U-shaped, each variable-section longitudinal beam comprises a web surface, an upper wing surface and a lower wing surface, and each upper wing surface and the corresponding lower wing surface are respectively bent along the upper edge and the lower edge of the corresponding web surface; and the lower wing surfaces, right above the rear axle, of the variable-section longitudinal beams are upward bent to form bent arch parts, each bentarch part comprises an arc-shaped curved surface which is upwards bent and a transition connecting edge connected with the lower edge of the corresponding web surface, and the transition connecting edges is an upward concave arc edge in the length direction. The tractor frame assembly with the variable-section longitudinal beams and the forming process of the variable-section longitudinal beams not only can reduce the height of a saddle from the ground, but also ensure the stress requirement of the longitudinal beams.