A theoretical investigation concerning the effect of arterial constriction on the flow of blood through a porous vessel in the presence of externally applied magnetic field has been carried out. In this study blood is assumed to be homogeneous Newtonian fluid flowing through a large blood vessel. In order to solve the problem, Adomian's decomposition method is employed for obtaining its analytical solution. The analytical expressions for stream function, axial velocity component, wall shear stress are obtained and the corresponding numerical solutions for a specific set of dimensionless parameters have shown graphically. The results obtained for different values of the parameters show that the flow is appreciably influenced by the presence of magnetic field as well as the porosity of fluid medium. Thus the method developed here can capture small changes in the complex flow domain.