Tight oil reservoir has narrow and small pore throat and strong heterogeneity. There is a high capillary resistance between oil and water two phases，so the water phase will firstly break through the pore throat with the lowest capillary force in the water flooding process. Along with the continuous breakthrough of the larger pores，a water channel will be formed gradually，and then the injected water will flow along the channel and lose the effect of oil displacement. This phenomenon will be more obvious when there is a series of micro-cracks in the reservoir. Pore structure of reservoirs and possible capillary force distribution during the water flooding in the tight oil reservoir were researched. According to pressure drop features of oil-gas-water capillary force during WAG，a new WAG method was proposed using the injection of trace gas slug and large amount of water slug to plug the natural cracks and larger pores in the tight oil reservoir. A new trace gas injection experiment was designed，and the injection intensity，injection cycle and injection times were optimized. The changes of the recovery efficiency were observed. The results show that trace gas-altering-water method is effective for enhancing development effect in the tight oil reservoir after the formation of water channel in water flooding.