Based on the mathematical model of resonant-loading fatigue test system, the prototype function of dynamic-loading was derived. With reference to the test data, the error source of loading-calibration has been analyzed. As a key point, error in strain measurement was estimated in detail. Furthermore, a modified prototype function was derived which was suitable for the fitting of dynamic test data. The number algorithm was also given. Lots of practical examples showed that the function type met the regular of resonant-loading well with a strong adaptability and high accuracy. It is better than other fitting method.