Abstract: Cavity is a common defect in concrete filled steel tube(CFST) and it directly influences the durability and reliability of CFST. In order to find out the internal state of CFST, the empty thickness can be estimated by using ultrasonic inspection(UT) method. Based on combination of sound time from UT and test performance of in-site drilling, the data was processed by line-fitting method, which could justify the flaws in the CFST accurately and quickly. This new method had practical application in engineering, and had certain reference value.