Tight oil reservoir has tiny pores so the imbibition effect is obvious. In order to enhance efficiency of waterflooding for tight oil reservoir, the method of combining water flooding and soaking was proposed. In order to clarify the mechanism of enhanced oil recovery of this new method and its main influencing factors, the impact of imbibition effect was considered and a model was proposed for imbibition. Combining this new model and the numerical simulation model for waterflooding, the numerical simulation for the water-flooding-soaking method was built. The results show that the effect of imbibition increases the swept volume of reservoir and displaces crude oil from the matrix, which improves the production efficiency. The main geology factors affecting oil recovery include matrix/fracture porosity, permeability and coefficient of imbibition. The main engineering factors affecting oil recovery include injection pressure, fracture interval and half-length. The field case also shows that this method can enhance production rate, its rate is twice time of the one by water flooding, the efficient period is about 200 d and the cumulative enhance oil is about 340 t. It indicates that this new method can enhance recovery rate of tight oil reservoirs in China and the built model can optimize parameters of this new method.