Abstract: Based on the acoustical theory and the simplified acoustoelastic equation for Lcr wave, ultrasonic stress measurement installation was established which consisted of a special transducer and an oscillograph with special higher digitizer board.The Lcr wave which is sensitive to stress is suitable to measure welding residual stress.Acoustoelastic coefficient of longitudinal critically refracted waves propagating in Q235 steel was obtained by tensile testing machine.The transverse stress of twin wire welded plates was measured by this system.The result fluctuation was discussed.The measurement process is real-time and quick which can be as the base of realtime reliability evaluation system for welded structure.