Based on the bubble dynamics, the generation and separation of bubble plays a leading role in the heat transfer of boiling condition. Before the bubble separation, micro liquid layer below bubbles is evaporated to dryness and heat transfer deterioration is the ultimate cause of boiling crisis, proposed a critical heat flux mechanism in the conditions of low velocity and narrow channel. A critical heat flux mechanism model is established. By calculation, in the low mass flow range the mechanism model has a high precision to predict CHF in a larger pressure and export dry degree ranges. Using the model the relationship of parameters is studied.