A method for and system of intrusion detection by using ultrasonic signals are provided. The method includes a training phase and a detection phase. The training phase includes capturing multi-echo signals when no intrusion is present under surveillance. The captured ultrasonic multi-echo signals are analyzed and features are extracted, learned and modularized. The detection phase includes a continuous collection of ultrasonic multi-echo signals. The captured signals are analyzed and features are extracted to compare with the features learned during the training phase. When intrusion is present, the features will be substantially different from the features learned in the training phase. Thus, an intruder can be detected. Experimental results indicate that when MDR is less than 0.1%, the FAR would be less than 2.5% for the invention. Therefore, this invention is applicable to intrusion detection.