Abstract: The main purpose of the study is to enhance oil recovery in the upper portion of a positive cyclothem thick water flooding reservoir formation by gas injection. Laboratory tests on a physical model were performed. The mechanism of three phase flow in physical models with different geological features was investigated. The movement and vertical distribution of gas, oil and water controlled by gravity, viscous force and capillary force were observed. This investigation shows that the result of gas injection depends on the geological features of the oil reservoir and the gas-liquid ratio at the outlet of the physical model can be taken as a measure of gas injection performance. Another large-scale physical model and some apparatuses for measurement and separation were developed in the course of study to provide a new technique for three-phase flow test in the future. A close agreement was obtained between model investigation and numerical simulation. The conclusion in the paper provides valuable information for gas injection field pilot test in Daqing Oil Field.