Abstract: Void fraction is one of the basic parameters for the study on two-phase flow. For flow boiling conditions in the narrow channel, it is difficult to directly measure void fraction by using probes. Through high-speed photography, a measurement method of void fraction at the exit of the narrow channel was proposed in this paper, which was based on the two-step morphological image method. This method can solve the problem of recognition for the bubble boundary at the flow channel and obtain the instantaneous values of void fraction. The method has good adaptability for various flow patterns. Comparing with the time-averaged results calculated by the energy conservation method, the relative deviation is less than 17%. The accuracy and reliability of the method are verified.