In this research, a Mach reflection and its effect on explosive free forming of confined cylindrical shells are studied numerically. This shells were manufactured from extruded 6063-T5 Aluminum alloy. The diameter of shell was 1.5 times larger than its length. Its ends were sealed with rigid sheets. The simulation of formation of Mach reflection and plastic response of shell were done with Autodyn hydrocode and coupled Lagrangian - Eulrian spatial discretization. Formation of Mach reflection occurred on end plates. It is observed that the generated pressure in an area that is affected by Mach stem is higher than elsewhere. This phenomena causes rupture in boundaries area of shell to plate connections, before forming process. The maximum of transverse deformation that obtained from this study compared with experimental results which done in explosion mechanic laboratory in K. N. Toosi university of technology. The experimental and numerical results shows more than 93% agreement. Meanwhile, because of blast waves reflection and interaction of waves, coupled Lagrangian - Eulrian method is suitable method for investigation of internal explosion problems. In addition failure modes was simulated with finite element software Abaqus and good agreement was found between the results.