Superconducting flux qubit is a promising candidate for quantum computing. When the superconducting qubit is driven by an oscillating field, it involves plentiful dynamics of macroscopic quantum states. I have extended the generalized Floquet approach to investigate multiphoton quantum interference in the superconducting flux qubit driven by intense ac fields. For resonant and nearly resonant multiphoton transitions, the generalized Van Vleck nearly degenerate high-order perturbation theory is extended for the analytic treatment of the Floquet Hamiltonian, which allows accurate treatment of the ac Stark shift, power broadening, time-dependent and time-averaged transition probability. For the plots of the time-averaged transition probability, the Bessel function-like fringe patterns are observed around multiphoton resonance positions [1], in good agreement with recent experimental results [Science 310, 1653 (2005)]. Applications of the Floquet theory to various superconducting qubits lead us to a better understanding of the results of spectroscopy measurement and the dynamics of strongly driven qubits.