Utilizing transverse flow CO2 laser, the influence of micro-fluctuation of laser power to processing quality is studied under high speed laser processing. The relationship between processing quality and processing velocity is analyzed, and evaluation of laser beam quality is discussed. The result indicates that the fluctuation frequency of voltage/current of high power transverse-flow CO2 laser is 300 Hz when stimulated by direct current power supply, which resulted in laser power fluctuation of 300 Hz. The influence of the fluctuation to processing quality can be ignored under lower speed processing, but the fluctuation will affect processing quality under high speed processing. Aiming at the processing of high quality and high speed, the evaluation standard of time-beam-quality should be brought up.