Abstract: For the safety evaluation of the fire hazard destroyed concrete, the traditional method uses the strength loss ratio to distinguish the fired concrete strength loss. The research work used the ultrasonic-rebound synthesis method to testing the strength change before and after high temperatures destroys, and it gained that there was no simplex linear relationship between the damage grade and the strength loss ratio, and that the traditional method showed a definite limination. The research work through investigating the “ultrasonic velocity-rebound value” curves fractal dimensions in the two dimensions plane, gained the corelation between the fractal dimension change and the high temperatures destroy time. It was concluded that using the strength loss ratio alone was able to distinguish the high temperature destroy time, and that using the fractal methods was more advantageous.