Abstract: Ultrasonic testing technology, with critically refracted longitudinal wave method as the core, is an important means for the nondestructive testing of residual stress in materials. Based on the acoustoelastic effect, this paper establishes a mathematical model for ultrasonic residual stress measurement, derives the calculation formula of measurement uncertainty, and systematically identifies the main sources of uncertainty throughout the whole testing process. In accordance with the specification JJF 1059.1-2012 of Evaluation and Expression of Uncertainty in Measurement, type A and type B uncertainty components are evaluated for key influencing factors including repeatability measurement, acoustoelastic coefficient, time-of-flight resolution of instrument, ambient temperature, and material differences. The combined standard uncertainty and expanded uncertainty are calculated, and a complete expression method for testing results is formed. The evaluation system proposed in this paper is both standardized and operable, which can provide a unified guideline for the reliability evaluation of laboratory and on-site ultrasonic residual stress testing results, and promote the standardization and large-scale application of this technology in the field of engineering inspection.