Abstract: In order to solve the ultrasonic testing technical problem of dissimilar steel small diameter tube nickel butt welding of austenitic steel and low alloy steel in thermal power plant, one new ultrasonic inspection method with small diameter tube adapted ultrasonic probe was researched by theory verification, experimental data and field experience. The results showed that the degrees of the influences of different defects on ultrasonic testing parameters were different. Better sensitivity for the fusion zone cracking in dissimilar steel smalldiameter tube welding joint testing was obtained when K value of transverse wave probe was small, such as K1~1.5. And it is good that K value of transverse wave probe was big (K2~2.5) for internal defects in dissimilar steel smalldiameter tube welding joint testing. This detective means had good detection effect for various excessive weld defects, which could ensure the safe operation of power units, improve economic benefits. The detection method mentioned in this paper could give assistance to the ultrasonic testing of dissimilar steel smalldiameter tube welds.