The ab initio potential energy surface (PES) for the HSiOH cis-trans isomerization was generated by the modified Shepard interpolation method at the computational level of the complete active space self-consistent field (CASSCF) method. This isomerization has three reaction paths, i.e., one in-plane and two out-of-plane paths, so the reaction-path PES (RP-PES) has first been generated by setting reference points on these paths for the interpolation. In this RP-PES, there is an artifact of potential ridges between the in-plane and out-of-plane paths. By adding molecular configurations around potential ridges as reference points, the global PES has been much improved (the potential ridges have disappeared). Following trajectory simulations on this full-dimensional PES, the energy level shift of vibrational ground states due to tunneling was estimated by the semiclassical method. It is verified that there is a relatively large tunneling effect in this reaction, which is consistent with the experimental observations.