There is no united method to evaluate developed low-grade reservoir. Cash flow economic evaluation method is widely used in China and other countries. Cash flow method contains three different economic evaluation methods, they are VP, IRR and investment recovery period methods. In this paper, we evaluate a developed low-permeability sandstone reservoir and a developed middle-high permeability complex fault block sandstone reservoir with the cash flow economic evaluation method. We get the evaluation standard charts of the developed low grade big reservior, developed fault block reservior with middle-high permeability, developed fault block reservior with low permeability and heavy oil thermal recovery reservior. This new cash flow method lays theoretical foundations for evaluation of developed low-grade reservoir and other kinds of reservoirs.