Slow hydrogen absorption rate and low capacity of a metal hydride tank are the main bottlenecks restricting the practical applications of hydrogen energy. It is crucial to avoid of reducing system gravimetric/volumetric capacity by the additional components for enhancing the hydrogen absorption rate. In this work, we present a new method to increase the hydrogen absorption rate on the basis of the influence of particle radius and porosity to the performance of a LaNi5 tank. A model considering the effects of volume expansion on mass, heat transfer and kinetics is proposed and solved by the finite element method, which constructs the relationship of the hydrogen absorption rate of a metal hydride tank with particle radius and porosity. The results indicate that larger particle radius and higher porosity is of benefit to the hydrogen absorption reaction. Compared with the model without volume expansion, a slightly slower hydrogen absorption rate is given by the model with volume expansion, which is attributed to the decreased thermal diffusivity. The simulation results obtained by the model with volume expansion agree well with the experimental data. The LaNi5 tank is further optimized based on the model with volume expansion, from which the hydrogen absorption time at 90% of maximum hydrogen capacity can be reduced from 1394.12 to 474.83 s when the particle radius is 100 mu m and porosity is 0.63.