The invention provides a portable fixture for assisting the assembly and welding of transverse and longitudinal beams, which includes a supporting beam, a jack and a clamping mechanism, the supporting beam is arranged on the top of the jack and is perpendicular to the jack, and the clamping mechanism includes two claws provided with folding hooks , the two claws are rotatably connected with the support beam respectively, and the two claws are separated from the two sides of the jack in the form of folding hooks, and a clamp for clamping the T-shaped bar is formed between the two claws. Based on the above structure, the fixture can reduce the gap between the longitudinal beam and the panel, make the gap close to zero, and improve the welding quality of the longitudinal beam; at the same time, the fixture does not affect the continuity of the welding operation, which is beneficial to the operation of the welding robot , but also saves the work of making, dismantling, and grinding the diagonal brace, which improves the welding efficiency and saves costs; in addition, the fixture is easy to operate, safe, low in labor intensity, compact in overall appearance, and can be used repeatedly , can be produced in batches, easy to promote, and has high economic efficiency.