Abstract: This paper studies residual oil distribution and crude oil property variations in waterflooding heavy oil reservoirs, and analyzes the main causes of crude property variation. The mechanism of enlarging swept volume through steam-flooding is further discussed and the principles for selecting the opportune time to change waterflooding for steam flooding is also presented. The operability of this technique is improved through analyzing the effects of steam flooding on development effectiveness and the points for attention when performing steam flooding at the late development stage of waterflooding heavy oil reservoirs.