In view of the change of ultrasonic incidence angle caused by the online ultrasonic dynamic detection and the low measurement accuracy, a nonlinear probability model of two-parameter ultrasonic flooding detection error correction is formulated. The error introduced by ultrasonic incident angle is compensated by the function approximation theory. Using the combination of basis function and third-order Lagrange interpolation, the ultrasonic incidence angle and the nonlinear correlation between the incident angle and the detection error are obtained. The incident angle of the nonlinear probability model and the ultrasonic distance are iteratively calculated. The incidence angle is used to solve the refractive angle in reverse, and the uncertainty of the ultrasonic incidence angle in the detection is resolved. In the range of 30 to 45 mm, the results show that after the model compensation treatment, the ultrasonic wave at 0° ~ 8° angles of the incident, the plate detection accuracy is 1% , which provides a basis for effectively improving the corrosion accuracy estimation.