Aimed at a large-opening specimen of composite fuselage panel from some kind of aircraft, the boundary conditions of mould design were determined from the aspects of stiffness requirement, use requirement and cost requirement of mould. In order to meet the boundary condition requirements of mould design, a square steel-pipe frame mould for composite panel forming was proposed. In the design of the mould, at first the mould material “INVAR” steel was determined with comparing the advantages and disadvantages of a variety of commonly-used mould materials. Then the structure layout and structure dimension parameters of the mould were introduced. By means of finite element simulation, the overall stiffness of the mould and the strength of the lift-condition were simulated and calculated. The calculation results show that the overall stiffness of the die is good and can meet the design requirements. Compared with traditional sheet-steel frame mould from the overall stiffness of the mould and the weight of the mould, the square steelpipe frame mould has greater advantages in composite panel forming.