Using the first-principle method, we investigated the effect of tungsten carbide (WC) surface layers on hydrogen retention in W. The results showed that C is not capable of capturing H atoms efficiently because of Coulomb repulsion between negatively charged C and H atoms in tungsten. Meanwhile, the repulsive force enhances the migration barrier for H to travel through WC layers. The high barrier, therefore, prevents the implanted H from escaping the W matrix. With H implanted continually, more and more H atoms are accumulated in the bulk and diffuse far from the WC layer, which contributes to blister formation in W.