Abstract: Based on thin-wall vessel theory and null-unloading-residual-strain assumption, a simplified engineering formula for autofrettage pressure of composite overwrapped pressure vessels (COPV) was deduced, in which the effects of bilinear isotropic and kinetic hardening behavior for metal inner were considered respectively. The validity of the engineering formula was verified through comparing with FEM. As an example, the residual strain field after autofrettage technique and corresponding stress field under the service loading case for a typical COPV were studied. The results indicate that the load capacity of COPV significantly increases by means of the autofrettage technique provide in this paper.