Here, we present the first experimental demonstration of topological phase transition within the generalized Brillouin zone, occurring in complex momentum space, through a one-dimensional non-Hermitian electric circuit lattice. By adjusting the boundary conditions, specifically the onsite potentials and couplings of boundary nodes, we observed the emergence of topological boundary modes in complex momentum space. Furthermore, we identified that this topological phase transition, known as an exceptional transition, is accompanied by the manifestation of an exceptional point. Our experimental findings hold significant promise for advancing the understanding of bulk-boundary correspondence within non-Hermitian systems by highlighting the novel topological phase transition that occurs in complex momentum space.