Abstract: Large diameter pipeline is subject to large stress internally during the operation and consequently its performances (e.g. corrosion resistance and fatigue resistance) are seriously impacted, so it is crucial to carry out online monitoring on its internal stress. In this paper, the two-dimensional plane stress testing system using ultrasonic longitudinal elastic wave and the membrane stress test urging unit were designed. Then, acoustoelasticity effect experiments were carried out under uniaxial stress state and biaxial stress state according to the acoustoelasticity theory of ultrasonic longitudinal elastic wave under uniaxial stress state and biaxial stress state to verify the accuracy of the theory. At present, the acoustoelasticity theory under uniaxial stress state is mainly adopted in the engineering to measure the stress distribution on the pipeline. It is shown by comparing the acoustoelasticity equation under uniaxial stress state with that under biaxial stress state that the effect of the included angle between stress direction and acoustic wave propagation direction on the stress measurement is ignored in the acoustoelasticity equation under uniaxial stress state. Therefore, a series of experiments were conducted to explore the influence laws of the included angle between stress direction and acoustic wave propagation direction on the stress measurement.