Abstract: The calculation formula of the resonance length of the dog-bone-shaped thin plate specimen with middle equal section for ultrasonic fatigue test was obtained by analytical solution, meanwhile the distribution functions of the displacement and stress amplitude along the resonance length were given. Considering weld reinforcement or not, finite element models of thin plate specimen and thin plate butt specimen were built respectively, and then modal results were analyzed. The results showed that the relative frequency error of the analytical method is less than 1%, while the frequency error caused by the weld reinforcement increased by about 1.7%. Moreover, the thin plate butt specimen of low-alloy steel Q345 was designed based on the above method, and then ultrasonic fatigue tests were conducted to verify the accuracy of the resonant frequency.