For efficient calculation of neutronic-thermal hydraulic coupling in pebble bed high temperature gas cooled reactor,a modified nodal expansion method (MNEM) was developed to solve convection-diffusion equation in cylindrical geometry.Within the framework of nodal expansion method,up to third-order polynomials combined with an orthogonalizd exponential function were adopted as basis functions for r-directed transverse integrated equation.Numerical tests reveal that the MNEM posses inherent upwind characteristic,and has good accuracy and stability for coarse meshes.