In substitution of path integral isomorphism of the quantum particle, an effective polymer ring model is proposed in the density functional calculation for hydrogen adsorption in single- walled carbon nanotubes. The excess intrinsic Helmholtz energy for quantum particles includes contributions from hard- sphere repulsion, interatomic bonding and soft attraction. The first two contributions are considered through the method developed by Yu and Wu [ J. Chem. Phys. 117, 2368 ( 2002)], and the last contribution is obtained from mean field approximation using Weeks Chandler - Anderson potential. The theoretical predictions are in good agreement with Monte Carlo simulation data for the density distributions of the hydrogen molecule inside the tube. In addition, the proposed model is applied to the calculation of the adsorption isotherms of hydrogen at 100 and 150 K. The present model is simpler than the current existing theories for quantum fluids. (C) 2003 American Institute of Physics.