Low permeability reservoir is characterized by poor physical properties, heavy heterogeneity, natural micro-fracture and connectivity anisotropy etc. The result will not conform to the real value if we simply calculate the single well controlled reserves by the conventional way of dividing the oil area. According to theory of flow unit dividing and the characteristics of low permeability reservoir, the flow unit method is put forward. It is to divide the flow units, and then calculate the single well controlled reserves regarding the flow unit as the calculating unit, finally, compare and analyze the results from this method, single well controlled area method and dynamic pressure drawdown method respectively. The comparison result proves the scientificity and practicability of the flow unit method.