The invention discloses a flexible assembling and welding tool for frames. The flexible assembling and welding tool comprises a platform, side beam compressing devices (2), cross beam positioning devices (3) and cross beam jacking devices (4), wherein the side beam compressing devices (2), the cross beam positioning devices (3) and the cross beam jacking devices (4) are arranged on the platform. The platform (1) is provided with first sliding grooves (11), and the side beam compressing devices (2) are arranged in the first sliding grooves (11) correspondingly through connecting pieces. Second sliding grooves (12) are formed in the middle portion of the platform (1), and the cross beam positioning devices (3) are arranged in the second sliding grooves (12) correspondingly. The cross beam jacking devices (4) are arranged between the cross beam positioning devices (3) side by side in the Y direction. The assembling and welding tool has extremely high universality and applicability; and when the tool is used for assembling and welding truck frames of different types, it is not required to change the tool, only appropriate adjustment is needed, the product preparation period is short, the product exchanging efficiency is high, the output capacity of a workshop of unit area will not be affected, and too large pressure will not be brought to the workshop for storing the tool.