Abstract: A dynamic spot diagnostic system has been developed to quantitatively measure the instability of a Continuous-Wave Chemical Oxygen Iodine Laser(CW-COIL). The system can measure the real time changes in drift, vibration of low frequency, intensity profile and spot diameter for the CW-COIL laser beam. The cause that induces the instability of laser beam was tentatively analyzed in this paper. The experimental results provided a basis for improving the COIL laser and for studying the laser-material interactions.