Abstract: Through drilled straight hole across the wall of minor diameter pipe (the outer diameter is less than 100 mm), tested ultrasonic signal of the straight hole by phased array ultrasonic testing technology, and studied the reflection rule. Experimental results show that the distance-amplitude curve of straight hole has similar variation tendency to the characteristic curve obtained from horizontal hole in national standard, but exhibits better applicability. The designed straight-hole is capable of revealing reflection character of minor diameter pipe. Hence, the sensitivity curve calibrated by the straight hole should provide high detection accuracy for the phased array ultrasonic testing of minor diameter pipe.