This study has discussed factors that influence the precision of conventional reservoir numerical simulation, including the accuracy of reservoir description, relative permeability curve selection, dynamic data entry, and grid precision. To improve the precision,many methods were used including staged numerical simulation, relative permeability curve in stages assignment, endpoint value calibration, grid scale processing in high water cut period, subdivided layers water injection. Compared with the traditional numerical simulation technology, the staged numerical simulation technology is more reliable, especially when the oilfield enters the extra high water cut development period with highly scattered remaining oil distribution. By using staged numerical simulation, the distribution of remaining oil saturation field is more accurate which can facilitate the late development of oilfield.