The launch box cover should meet the requirements of light weight and compact structure. In order to meet the performance requirements， a new design scheme named non-penetrating interval slit composite fragile cover is proposed. Finite element models of progressive damage under pressure and break are established， respectively. The influence of structural parameters on mechanical properties of the new structure fragile cap is analyzed， and the damage failure mechanism of fragile cap break is discussed. The results show that the finite element model established in this paper can effectively predict the maximum deformation of the cover under pressure and the bursting pressure of the cover under bursting condition. The increase of the cover thickness will lead to the decrease of the maximum deformation and the increase of the breaking pressure of the frangible cover under the pressure condition. The increase of the slit width has little effect on the maximum deformation of the cover under pressure. However， the weak area of brittle cap will enter the damage failure stage earlier under the condition of breaking， which will have a negative impact on its breaking performance.