Abstract: Ultrasonic method is usually used to test the depth of fire-damaged concrete. It is assumed that the structural members could be simply classified into damaged layer and undamaged layer in the concrete and the damage depth can be calculated with a series of single-sided ultrasonic measured data. This method is simple and convenient but has a less precision in the practical application. A new improved algorithm was introduced in this paper. It is based on the assumption that the damage model was hyperbolic and the propagation path of the first wave was parabola. ABAQUS finite element analysis software was used to simulate the ultrasonic wave propagation in the damaged concrete for theoretical proof of the improved algorithm. The result showed this improved algorithm owned higher accuracy and better feasibility.