The grain size of metal materials is an important parameter for the characterization of mechanical properties. Realizing the online detection of grain size plays an important role on improving product quality, upgrading intelligent factories, and intelligent scheduling. Firstly, the experimental platform of laser ultrasonic online nondestructive testing based on the thermoelastic mechanism was introduced in this paper. Then the modeling methods characterizing grain size based on frequency domain energy attenuation, and the grain size and distribution based on spectral attenuation, were analyzed. Finally, the experimental simulation of some environmental disturbance was carried out. The online detection of grain size based on laser ultrasonic technology is realized, and the detection accuracy can reach more than 86%. When the sample movement is within 20 mm/s, the out-of-plane jitter is ±7 mm and the deflection angle is within 0.8°, the detection results can be effectively ensured.