Laser spot thermography(LST) is a novel remote nondestructive testing technology. It can efficiently detect surface cracks. In this paper, a fast simulation method based on frequency domain summation and database strategy is developed to simulate the heat flow generated by the laser spot source and to investigate the relationship between crack size and temperature distribution. First, the feasibility of the new method is validated through comparison with a conventional method for both the time and the spatial domains. Second, suitable characteristic parameters are introduced, and a reconstruction algorithm based on a deterministic inversion method is developed. Finally, an infrared thermography testing system is established, and LST signals of surface cracks in a plate specimen are measured. The feasibility of the inversion method is validated by the accurate reconstruction result of crack size.