Abstract: Considering the structural characteristics of flow passage in the rectangular narrow channel and the annular flow predicted analytical model of Critical Heat Flux（CHF） in round tube, the annular flow CHF analytical model in rectangular narrow channel has been obtained. This model can be used to predict the CHF of boiling two-phase flow annular flow in rectangular narrow channel which gap width is not less than 0.5mm. The analysis and calculation indicate that when the inlet width-gap width ratio of rectangular narrow optical channel is 25～85, the enhancement of CHF in channel is obvious. At the same time, considering the characteristics of two-phase flow, the new determinant laws of CHF in boiling two-phase flow system has been derived. The analysis and calculation indicate that this determinant laws is appropriate. The best width-gap width ratio of heat transfer enhancement is 45～75. Based on the above results, the reference width-gap width ratio of heat transfer enhancement is 45～75 in rectangular narrow channel.