The invention discloses a positive and negative assembly welding clamping fixture for an aluminum alloy roof of car and is characterized in that the invention is provided with a plurality of positive and negative assembly conversion clamping fixture units driven by a connecting rod for a cylinder link gear; the positive and negative assembly conversion clamping fixture units includes a rotating shaft, a positioning supporting plate, a rotor sleeved on the rotating shaft, and a positive clamping fixture plate and a negative clamping fixture plate fixed on the rotor; the positive clamping fixture plate and negative clamping fixture plate are vertical to each other and parallel with the rotating shaft. Welding craft equipment is designed with a plurality of the positive and negative assembly conversion welding clamping fixture units, namely the positive and negative surfaces of the craft equipment consists of the positive clamping fixture plates and the negative clamping fixture plates of the clamping fixture respectively and converses the positive clamping fixture plates and the negative clamping fixture plates of the clamping fixture through a cylinder interlock mechanism in turn. The invention finishes the positive and negative clamping compound welding on the welding of units in turn through the cylinder link gear. The positive and negative assembly welding of aluminum alloy roof of the car are completed by one set of craft equipment, thus saving the cost of the craft equipment cost, reducing the cost, occupying few a manufacture state for an automatic welding machine and improving the manufacture efficiency.