A tree-type cylindrical-shaped nanoporous filtering membrane is optimized with two levels of branches and a lot of branch pores. In this membrane, the branch pores are parallel with their trunk pore and their radius Rb,1 is determined by the filtration requirement; The radius of the trunk pore is optimized according to the radius of its branch pore for achieving the lowest flow resistance of the membrane. The calculations were made respectively for the optimum ratios of the radius of the trunk pore to the radius of the branch pore and for the corresponding lowest flow resistances of the membrane for different operational parameter values. It was found that for given operating conditions, the increase of the number N of the branch pores significantly increases the optimum ratio of the radius of the trunk pore to the radius of the branch pore but significantly reduces the corresponding lowest flow resistance of the membrane; However, an over large N is not beneficial owing to not obviously dropping the flow resistance of the membrane; The choice of the number N of the branch pores may heavily depend on the liquid-pore wall interaction. It was also found that for any given N, this membrane has a good capability of liquid-liquid separation when Rb,1 is below 2nm, since its flow resistance for the liquid with a strong interaction with the pore wall can be more than 100 or even more than 1000 times that for the liquid with a weak interaction with the pore wall.