This paper presents the behavior of pressure in an air–water shock tube. In this work, high-pressure air (at 100 bar) interacts with water (at 1 atm ∼ 1 bar) through an orifice in a 100 mm constant diameter tube. The experiments are repeated with three different orifice plate diameters of 4, 8, and 15 mm. The variation of pressure during the transient stage is recorded in these experiments and it is found that with increasing orifice diameter, the amplitude of the pressure increases linearly with time when all other conditions are unchanged. The same phenomenon is simulated using the ls-dyna® software using an arbitrary Lagrangian Eulerian (ALE) method to solve the problem numerically. Simulations are made with a range of orifice diameters. The experimental results confirm the validity of the simulations algorithm. The simulations also demonstrated that the pressure behaves linearly with orifice diameter only when orifice diameter is less than 15% of the tube diameter.