The flow field of oil?Water in tee tube by using mixture multiphase model was studied, in addition to study the separation efficiency of tee tube response to the ratio of oil at inlet，the split ratio at outlet, the velocity of inlet, the length of branch tube and the length of main tube. The result show that: oil gathered and separated at the intersection of branch tube and main tube. The reason of oil gathered at main tube was caused by the gravity, and at branch tube, oil gathered phenomenon was mainly due to the centrifugal separation caused by the density difference between oil and water. Oil content at main entrance significant influence on separation efficiency of tee tube, and separation efficiency reduced with the increase of inlet oil content, so tee tube separator suit for separating the oil-water which has low oil content. Study result shows the efficiency would be increased when increased the split ratio, that means that can improve the separation efficiency by mounting throttle at outlet of main tube. The separation efficiency is not sensitive to the length of branch tube and outlet of main tube, so it can be only in accordance with the requirements of the manufacturing and installation in tee tube design process.