Abstract: In the application of cancellous bone ultrasound diagnosis based on backscattering method, it is of great important to estimate fast and accurately whether the valid backscattering signal exists in the received signM. We propose a fast estimation method based on spectrum entropy. With 984 records of adult calcaneus clinical data, we estimate the validity of the backscatter signal using this method. The results of our method and the results of experience- base judgement were compared and analyzed. And the two key parameters, the signal range length and the segments number of the spectrum entropy, were analyzed. Analysis result shows when the signal range length is 13 μs and the segments number is 15-20, this method can get the best result (accuracy〉95%, sensitivity〉99%, specificity〉87%), while taking little calculation time (15 ms). Therefore, this spectrum entropy method can satisfy the accuracy and real-time requirements in the ultrasonic estimation for cancellous bone