Abstract: The void fraction and interfacial area concentration are important phase interfacial parameters in two-phase flow. Accurate acquisition of void fraction and interfacial area concentration under churn flow and annular flow in a narrow rectangular channel is the key to construct and improve the two-fluid model. In this paper, a visualization experiment was conducted to study the flow characteristics of gas-liquid two-phase flow in a narrow rectangular channel with a cross-section of 65 mm×2 mm. The range of gas-phase superficial velocity is 1.9 m/s and liquid-phase superficial velocity is 0.1-1.5 m/s. Flow pattern includes churn flow and annular flow. Based on the idea of gas-phase splitting calculation, an analytical calculation method to obtain the void fraction and interfacial area concentration under the churn flow and annular flow was proposed using high-speed camera. And the calculation method for the local parameters in the direction of the wide edge of the narrow rectangular channel was also proposed. For the two-phase flow video under each experimental condition, picture extraction and data processing were performed at 0.1 s time interval, and the duration of data processing is 60 s for each condition. The relative deviation between the calculated void fraction of the empirical formular and experiment value in the narrow channel is within 10%. The correlation coefficient K between interfacial area concentration and void fraction was proposed. As the void fraction increases, the value of K gradually converges to 1 from greater than 1. This result is consistent with the theoretical analysis and verifies the rationality of interfacial area concentration calculation method. To verify the accuracy of the local parameter calculation method, the experimental images were divided into 10 equal parts along the broad side of the narrow rectangular channel. The relative deviation between the average values of the local void fraction and interfacial area concentration and the results calculated without partitioning is within 5%. The calculation method can be helpful for the study of phase interfacial parameters in narrow rectangular channels under complex flow patterns.