Hydrogen in the steel wall can cause hydrogen embrittlement of the wall material and thereby change the carrying capacity of the vessel. A theoretical model of hydrogen diffusion in the steel wall of a high pressure vessel was established and the formula of hydrogen content in the steel wall was deduced. Based on the hydrogen content formula, the formula of hydrogen pressure within microvoids which naturally exist in the steel wall of a spherical pressure vessel was deduced. At last, as an example to demonstrate the meaning of solving hydrogen pressure in microvoids, by using a representative volume element (RVE) model to carry out FE numerical simulation, the effects of hydrogen pressure on equivalent mechanical properties of the wall material were studied. It is found that the higher the gas pressures are, the lower the ultimate strength, specific elongation and percentage contraction of area are, which is in good accordance with the phenomena of metal hydrogen embrittlement.