In this paper, a visualization investigation for the characteristic parameter (velocity, length, liquid film thickness, etc.) and pressure drop of slug flow in a narrow rectangular channel with a cross-sectional area of 68 mm × 1.9 mm was carried out under horizontal condition. The slug bubble velocity is linearly related to the two-phase mixture velocity. The slug bubble velocity of laminar flow zone (${\text{Re}}_{\text{l}\text{o}}\text{<}2500$) and turbulent flow zone (${\text{Re}}_{\text{lo}}\text{≥}2500$) was fitted, respectively. The slug bubble liquid film thickness was measured based on PCB (Printed Circuit Board) liquid film sensor. The experiment results show that the liquid film thickness increased with increasing Ca under ${\text{Re}}_{\text{b}}\text{<}3100$, while liquid film thickness approximately a constant under ${\text{Re}}_{\text{b}}\text{≥}3100$. A new correlation for predicting liquid film thickness was proposed and validated with 210 available data. Moreover, based on the present and published data, a new slug bubble length correlation was also developed. The slug frequency increases significantly with the increasing liquid flow rate and decreases slightly with the increasing gas flow rate. But the flow condition has little influence on the slug length. Existing correlations for pressure drop based on separated flow model were summarized and evaluated with the current data. Lee’s correlation perform well for the region dominated by surface tension(${\mathit{Re}}_{\text{b}}\text{<}3100$) while Chisholm’s correlation perform well for the region dominated by inertia force (${\mathit{Re}}_{\text{b}}\text{≥}3100$).