The N-k contingency constrained economic dispatch method guarantees the system stability and security while N-k contingency occurs. However, the introduction of N-k contingency leads to a higher operation cost. Flexible AC transmission system (FACTS) devices are adopted to control the power flow following N-k contingencies, which helps enhance system flexibility. An N-k contingency-constrained two-stage robust unit commitment with FACTS is thus proposed. Since this model is a two-stage tri-level mixed integer linear optimization problem that cannot be solved directly, a two-level solution method based on the nested column-and-constraint generation (Nested C&CG) is proposed. The upper level decomposes the primal problem into master problem and subproblem. The lower level is used to solve the subproblem and obtain the worst scenario. The proposed model and solution technique are tested on a six-bus and a modified IEEE RTS-79 system. The numerical results show the effectiveness of the proposed model and solution method.