The sheet metal hydroforming technique is easy to achieve lightweight and is widely applied in the aerospace and automotive fields. In order to realize the reliability test for uniaxial tensile forming properties of sheet metals under the condition of three-dimensional stress, experimental apparatus and dedicated fixtures for uniaxial tensile tests of sheet metals under fluid pressure are designed in this article. The stainless steel 304 is employed for this experiment, and commercial finite element software ABAQUS is also adopted to study the deformation behavior of sheet metals under fluid pressure. The results indicate that when the fluid pressure of gauge length reaches a certain value, the specific region will present "double necking" and the peak values of equivalent stress and reduction rate are much lower than that of the conventional ones. When the strain rate is 0.01 s^–1, the elongation of the 0.95-mm thickness sheet increases more obviously, reaching 24.1%. With fluid pressure, the fracture morphologies of stainless steel thin sheets and thick sheets are all ridge distributed from both sides to the center of symmetry, and tiny dimples distribute mainly in the rest parts. Based on the results of the experiment and finite element method, the uniaxial tensile forming properties of sheet metals under fluid pressure have been evaluated successfully and easily by this experimental apparatus.