The hydrostatic test of integrated multilayer clamping high pressure vessel is performed from 15.7 MPa to 56.52 MPa (1.8 times of the design pressure). The measured stress values have deviation from the Lamè equation (maximum error 34%). The inner wall stress is low while the outer wall stress is high. The layered vessel mechanics model is deduced to analyze the deviation. The inner shell is an external pressure vessel and the outer shell is the internal pressure vessel. The contact of the inner shell and the outer shell generates preload stress. The multilayer structure parameter is a universal nondimensional parameter to calculate the preload stress of layered vessel, and the corrected theory values have smaller error in high pressure (less than 10%). The super hydrostatic test alters the preload stress distribution and the gaps between layers.