The invention relates to a bidirectional through-flow turbine for generating power by tide. The bidirectional through-flow turbine comprises a forward inflow runner, a vertical shaft, a supporting frame, a bidirectional power generation rotating wheel, a reverse inflow runner, a generator and a speed increaser. The bidirectional through-flow turbine is characterized by further comprising front equal-width movable guide vanes and rear equal-width movable guide vanes, the front equal-width movable guide vanes are arranged between the forward inflow runner and the bidirectional power generation rotating wheel, the rear equal-width movable guide vanes are arranged between the bidirectional power generation rotating wheel and a reverse inflow runner, the forward inflow runner has a forward elliptical section, the ratio of the short axis to the long axis of the forward inflow runner is a golden division ratio, the reverse inflow runner has a reverse elliptical section, and the ratio of the short axis to the long axis of the reverse inflow runner is a golden division ratio. By taking double measures of the elliptical runners and the front and rear symmetric equal-width movable guide vanes, the composite effect of the operation efficiency of the turbine is greatly improved, the overall power generation efficiency of a generator set is remarkably improved, and the bidirectional through-flow turbine is applicable to replacing various through-flow turbines of low-head tidal power stations.