Pipeline-type oil-water separation technology has the advantages of small footprint, high separation efficiency and low cost. A Y-type tubular oil-water separation structure was designed based on the principle of pipeline-type oil-water separation. The effects of internal flow field and external parameters on separation efficiency of Y-tube were studied by adopting standard k-ε turbulence model and mixture multiphase flow model in Fluent numerical simulation software. The results showed that the Y-shaped tube had little disturbance to fluid, stable internal flow field characteristics and was not easy to generate eddy current, which could realize oil-water separation. There was oil-water separation at the junction of branch pipe and main pipe. The oil-water separation efficiency increased gradually with the increase of the oil droplet size. The larger of the oil droplet size, the better the oil phase will float and gather in the upper layer of Y-shaped tube, which effectively improved the oil-water separation efficiency. The inlet velocity had a great influence on the oil-water separation effect. The higher the inlet velocity, the shorter the oil phase floating time, and the lower the separation efficiency of Y-shaped tube. The separation efficiency increased with the increase of branch shunt ratio. When the inlet velocity was 0.5 m/s, the separation efficiency would not increase when the shunt ratio exceeded 0.4. The results provided a new idea for oil-water separation and a theoretical reference for the design of Y-shaped tube.