New hydraulic expansion technique was invented, and conjoint results were also estimated. High pressure reactor’s overall water pressure test is used to expand lining plastic deformation, and then eliminate the gap between lining and shell. According to elastic-plastic theory, the mechanics model was induced. The conjoint result was estimated by measured layer gap, stress and strain. The results show that theoretical calculation is basically consistent with actual results, the biggest error is 13.8%; although the biggest preliminary gap reaches 16mm, remnant gap is only 0.6~0.9mm after hydraulic expansion; during pressure increase test, shell material always keeps elastic. Lining and shell almost become an integrated material after the initial pressure more than 3.1MPa, the linearity between cylinder stress and initial pressure is more than 0.99.