Under condition that all processes of on-the-spot data collection comply with technical specification strictly,the reliability of test result by ultrasonic-rebound combined method is mainly dependent on the accuracy of the strength curve itself.With Shepard interpolation model a kind of new method based on the similarity principle to calculate concrete strength at detecting region by ultrasonic-rebound combined method was introduced.The method is directly based on the sampled data and used to find the concrete strength by nonlinearly combining several results of concrete strength from sample set.The samples to be selected by the model as input in the combination were decided by the similarity comparing with the predicting object in the detecting parameters.An example shows that the predicting accuracy of the method in view of relative error as evaluation index is better than that of formula in the technical specification for detecting strength of concrete by ultrasonic-rebound combined method(CECS 02:2005).