Abstract: This paper analyzes the principle of ultrasonic detection of concrete internal hole size,selects and uses the NM-3C type non-metal testing analyzer,and processes concrete internal hole size detection test by using contralateral test method,the results show that it is feasible to detect the concrete of 4-10 cm hole with ultrasonic method;But for the holes which less than the probe size,the error is bigger;for more than 10 cm holes,still need further verification.