This paper proposes a double porosity model to deal with the mass transfer between capillaries and tissues in the lung microcirculation on the basis of Ref. [4]. Utilizing the analytic method of small parameter expansion and the numerical method, the nonlinear governing equations are solved. Comparison shows that the small parameter solution is accurate enough to illustrate the mass exchange between the lung capillary and tissues. Finally the results are used to discuss the mechanism of formation of lung edema. This paper proposes a double porosity model to deal with the mass transfer between capillaries and tissues in the lung microcirculation on the basis of Ref. [4]. Utilizing the analytic method of small parameter expansion and the numerical method, the nonlinear governing equations are solved . Comparison shows that the small parameter solution is accurate enough to illustrate the mass exchange between the lung capillary and tissues. Finally the results are used to discuss the mechanism of formation of lung edema.