The relevance. Compared with conventional reservoir, offshore heavy oil reservoir faced the problem of lower water flooding recovery. It is helpful to improve the water flooding recovery of heavy oil reservoir if we have more knowledge about seepage characteristics of water and oil. The main aim of the research is to investigate the change rule of seepage characteristics of water phase and oil phase in offshore heavy oil reservoir, which is helpful to the decision of field construction and can provide guidance for numerical simulation. Methods. The reservoir geological characteristics, fluid properties and field construction technology of the destination offshore heavy oil reservoir in Bohai Oilfield were taken into consideration. The impact of core permeability, injection water washout and crude oil viscosity on relative permeability curve and displacement efficiency of water flooding were studied, through core displacement experiment using displacement method of unsteady state. The change of core inside structure is presented by core CT scanning and mercury pressure test. Results. With the increase of core permeability, water phase relative permeability increases, the swept volume of water phase becomes bigger, two-phase flow span gets wider, and ultimate recovery enhances. The function of «cleaning» and «erosion» that water washout exerts on core pore structure would enlarge the pore throat radius, increasing core permeability, and its influence on relative permeability curve and displacement efficiency is the same as the influence of permeability increase. With the crude oil viscosity increasing, mobility control ability of water gets weaker in the process of water flooding, displacement phase viscous fingering phenomenon aggravates, the swept volume of water phase becomes smaller, two-phase flow span gets narrower, and ultimate recovery decreases.