Abstract: Proved reserves of oil is a variable influence by the changing of available recovery technology and economic conditions. There are seven steps on calculating proved reserves of oil using cashflow method: ①Geologic evaluation of the reservoir. ②Reservoir engineering evaluation. ③Investment estimation. ④Determination of costs and expenditure. ⑤Sales revenue estimation. ⑥Sales taxes and fees estimation. ⑦Make the cashflow statement. Based on the calculation of the proved reserves of the water flooding light oil field and water flooding high pour point oilfield both on the eastern oil bearing area, we further proved the feasibility of the cashflow method on calculating the proved reserves of oil in different kinds of oilfields.