The structure of Mach shock disk (MSD) is analysed emphatically by constructing free jet model, adopting compressible liquid axial symmetric N-S equation and RNG k-ε model. Through the numerical simulation of free jet outer flow field under the outlet static pressure of 6.145×105 Pa, the paper describes the jet structures of different outlet Mach numbers, analyses the relation between MSD and outlet Mach number of nozzles. It reveals that the width of MSD presents convergent trend with the increasing of Mach number, and indicates that the outlet Mach number should be improved properly by ensuring disappearance of overexpanding waves under the working pressure. It is illuminated that the Mach number will be convergent to a fixed position with the increasing of outlet Mach number. The veracity of simulation results and the validity of conclusion analysis are proved by contrasting validating of flow field visualization to the solved model.