Laser-generated ultrasonic wave characteristics in the rail foot weld were simulated and reported for qualitative analysis and evaluation of sub-millimeter-level crack growth. Numerical analyses using the finite element method (FEM), the propagation characteristics, and displacement field distribution of a laser-generated ultrasonic wave after the interaction with cracks were fully demonstrated. By calculating displacement amplitude distribution, the optimal sensing position and area were the laser incident point and the upper surface, respectively. Crack growth degree toward the rail bottom and axial direction can be confirmed by analyzing time and amplitude of the echoes originating from the rail bottom and crack surface reflection. By combining time with peak intensity of the echo reflection from the rail bottom, the sub-millimeter-level crack growth process inside the rail foot weld is capable of acquiring and evaluating. The results justify that the laser ultrasonic technique, characterized by laser excitation and laser detection, is a competitive nondestructive testing technique for sub-millimeter-level crack growth evaluation and detection inside the rail foot weld.