In this paper, a method for detecting the internal stress of a steel plate based on laser ultrasonic guided waves is proposed. The experimental results show that a pulsed laser can excite broadband laser ultrasonic guided waves in the silicon steel plate, and the A0 mode is the main mode. The phase velocity of the laser ultrasonic guided wave increases with the increase of tensile force, while the group velocity decreases with the increase of tensile force. There is an obvious linear relationship between tensile stress and the time difference of head wave advance and wave packet delay of the laser ultrasonic guided wave signal. The laser ultrasonic guided wave testing method can be used to realize the non-destructive, non-contact, and high-precision detection of internal stress of a strip steel, and this method may become a new method for the on-line stress detection of strip steels.