Abstract: Based on the thermohaline data observed by an underwater glider in the Northwest Pacific Ocean from August to October 2019, this paper analyzes the variability of the mixed layer and discusses the dynamical mechanism accordingly. The results show that the temperatures of the mixing layer lowered gradually in accordant with the seasonal alternations, but the depth of the mixing layer increasesed. Further correlation analysis shows that the variations of temperature and depth in the mixing layer were mainly determined by the external atmospheric forcing (wind and heat flux). The phenomenon observed by underwater glider showed that the temperature in the mixing layer decreased abnormally and the depth became anomaly shallower. The quantitative analysis of the heat budget within the mixed layer revealed that the vertical enrolling, which caused by the meso-scale eddy and by the temporal variations is the main reason of the phenomenon. It can be concluded that the meso-scale eddy dominated the unusual occurrence of mixed layer.