The laser radius should be calculated with high precision when analyzing the characteristics of a Lamb wave excited by a laser in a thin plate. However, traditional methods for measuring the radius of a laser beam are complex. The paper aims to propose a novel convenient method, to the best of our knowledge, for measuring the radius of a laser ultrasonic beam. A laser interferometer (receiving laser) is used to receive ultrasonic signals excited by the laser to be measured (exciting laser) on the surface of a test block. Considering the characteristics represented by the thermal wave, positions where the receiving beam contacts and separates from the exciting beam are determined, and the spot of the radius is calculated by corresponding geometric relations. Experiments show that the accuracy of the proposed method is in the order of magnitude of 0.01 mm, and the relative errors of experimental data are within 2%. This paper provides an alternative method for the measurement of the laser beam radius and has great significance for the application of laser ultrasonic detection in the thin plate and the calculation and analysis of the dispersion curve.