The experimental apparatus which is applied to study the swelling of clay has been designed and made. The experimental apparatus is mainly composed of five parts which are(1)nitrogen tank, (2) gas valve, (3) pressure gauge, (4) temperature control box, and (5) pressure membrane system. The fifth part is made of polymethyl methacrylate and consists of two chambers, i.e.,upper chamber and lower chamber in which two Ag-AgCl electrode are respectively installed to indicate the equilibrium.The swelling of clay in the upton montmorillonite clay system with overlapping electric double layer was studied at 25℃ by using the method of Ag-AgCl electrod to set the equilibrium. The clay suspensions with different electrolyte concentrations were made up by montmorillonite and NaCl solution of given concentration. When the experiment began, the given pressure was applied to the clay suspension surface in upper chamber by nitrogen gas. When the difference of millivolt between two Ag-AgCl electrode did not vary with time, the equilibrium had been obtained. The experimental data, such as swelling pressure, P, distance between two clay plates, λ, Cl^- concentration is clay cake of the upper chamber and in the bulk solution of lower chamber were measured. The experimental results shown that the swelling pressure, P, was the exponential function of the distance between two clay plates, λ, and the variation of swelling pressure with the distance between two clay plates could be described by erapirical equation.