Abstract: The thermal recovery process is not appropriate to deep heavy oil reservoir development due to economic and technical constraints. Some domestic heavy oil reservoirs developed by conventional waterflooding process have achieved the better results. But with the crude oil viscosity increasing, the water flooding effect is getting worse and worse, and the recovery efficiency is lower than the related economic limit, this process is not suitable for such a reservoir. So viscosity is the key to determine whether such a reservoir is developed by waterflooding process or not. Wellblock Ji?7 in Changji oilfield of Junggar basin is a deep and large heavy oil reservoir with depth of 1 317~1 836 m and oil viscosity of 40~3 000 mPa·s. This paper makes field water flooding test and lab experiment to determine the limit of oil viscosity suitable for waterflooding process. It is suggested that the reservoir with formation oil viscosity less than 2 470 mPa·s can be developed by waterflooding process, which provides scientific basis for large?scale waterflooding development of Wellblock Ji?7 deep heavy oil reservoir