A multiscale theoretical method, which combines the first-principle calculation and a grand canonical Monte Carlo (GCMC) simulation, is used to predict adsorption capacity of hydrogen in silicon nanotube (SiNT) arrays at T=298 K in the pressure ranges from 1 to 10 MPa. In the multiscale method, the binding energy obtained from the first-principle calculation is used as an input in the GCMC simulation. It is found from the first-principle calculation that the SiNT arrays exhibit much stronger attraction to hydrogen both inside and outside SiNTs, compared to the isodiameter carbon nanotubes (CNT). The subsequent GCMC simulations indicate that the SiNT arrays present distinct improvements of 106 %, 65 % and 52 % in gravimetric adsorption capacity of hydrogen at P=2, 6 and 10 MPa, respectively, compared to the isodiameter CNTs. It suggests that the SiNT array is a promising candidate for hydrogen storage.