The invention belongs to the field of packaging box manufacturing, and provides a die for forming a paper box body and a die size adjusting method. The die comprises an upper die plate and a lower dieplate, wherein the upper die plate and the lower die plate are arranged in parallel and opposite to each other. First guide rods are perpendicularly fixed on the inner sides of the upper die plate and the lower die plate, the first guide rods are provided with thread holes in the length direction, the two ends of a first threaded rod are spirally arranged in the first guide rods on the upper dieplate and the lower die plate correspondingly and are reverse threads, a first turbine is coaxially fixed on the first threaded rod, a first worm is also matched and arranged on the first turbine, endplates are arranged at the two ends of the upper die plate and the lower die plate, the first worm is arranged in the length directions of the upper die plate and the lower die plate, and the ends ofthe first worm are rotatably arranged on the end plates. The problems that an existing packaging box forming die is complicated to replace, is large in labor intensity and greatly influences productivity and processing efficiency are solved.