Abstract: In order to study the accuracy of the calculation results of concrete crack depth by ultrasonic plane testing method in the Technical Specification for Inspection of Concrete Defects by Ultrasonic Method（CECS 21:2000）, the influence of range error on the calculation results of crack depth is analyzed based on the detection principle of ultrasonic plane testing method, and the method of curve fitting based on hyperbolic model is proposed to determine the crack depth. Combined with an example, the correlation coefficient test method is used to test the fitting effect. The example analysis shows that the calculation result by the method of curve fitting is basically close to the measured value, and more accurate compared with the method of CECS 21∶2000. The results of curve fitting method and finite element method are basically close. Theoretical analysis indicates that using smaller range can reduce the influence of range error on the calculating results of crack depth and the method proposed in the paper can effectively reduce calculation error under the conditions of smaller range.