Ultrasonic infrared lock-in thermography is a novel nondestructive testing technology, which mainly combines the modulated excitation and lock-in technology to achieve infrared thermography test. With the lack of study on the frictional heating simulation and interior heating mechanism during the defect heating and diffusion, the electric-force analogy method was utilized to build the finite element model of the ultrasonic transducer and the metal plate with the fatigue crack. Simulation results show that the crack heating periodically increases with the modulated ultrasonic excitation, and the heating area of crack faces is closed to the excitation side by the influence of the engagement force. Based on the simulation results and the Green function, a theoretic model was introduced to describe the heat diffusion of the crack frictional heating and the heat distribution of the crack vicinity was demonstrated. Further, the depth of the heat source was estimated with the ratio between top and lower surface of the test plate(P-value), which shows consistency between simulation and theoretic results. This study aims to enrich the theoretic basis in ultrasonic infrared lock-in thermography.