Abstract: In the traditional laser beam quality measurement method, an adjustable attenuation module is installed on the laser beam path in front of the change coupled device (CCD) camera target to attenuate the laser beam. However, the high-accuracy measurement is difficult to carry out due to the limitation of CCD camera pixel size by this method. On this basis, an evaluation method of laser beam quality with roller slit was proposed. In the measurement, the scanning slit on the slit roller was used to directly scan the measured laser beam, and the InGaAs detector was used to cooperate with the focusing lens for measurement. The high-accuracy incremental encoder was coaxial with the motor, which ensured the synchronous acquisition of position and data. According to the measured laser pulse frequency and the diameter range of the spot, the scanning frequency could be adjusted, and the resolution of spot spatial sampling was better than 1μm. The experimental results show that the value of laser M² factor measured by this method is consistent with that provided by the measured laser, and the measurement uncertainty is less than 10%.