The test of stress distribution and characteristics of pressure vessel is of great significance to ensure the safe and reliable operation of pressure vessel. For the rectangular vessels,the external stiffeners are used to improve their bearing capacity,so their stresses are more complex than that of cylindrical and spherical vessels. In this paper,a set of data acquisition system of stress test was self-developed for a rectangular vessel of desalination pilot plant by using the electrical resistance strain gauge method. All stress tests were carried out under the half-water and operation conditions,the Mises stress and its variation at different positions of the rectangular vessel was obtained. According to the measured strain data,the strength analysis was conducted. The results indicated that the Mises stress of the rectangular vessel under operation conditions is larger than that under the half-water condition in which the maximum Mises stress is 247. 09 MPa. The dangerous area of the rectangular vessel is mainly in the middle of the shell,and the stress on the edge of the shell is small. This study could provide an analysis method for a rectangular vessel of desalination pilot plant and the optimal design of rectangular vessels.