We study the effect of a strong and low frequency (ω < Δ, the superconducting gap) electrical field on a superconducting state. It is found that the superconducting gap decreases with the field intensity and wavelength. The physical mechanism for this dependence is the multiphoton absorption by a superconducting electron. By constructing the state of a superconducting electron dressed by photons, we determined the dependence of the superconducting gap on E / ω and temperature. We show that the critical temperature is determined by the parameter E / ω which is distinct from that induced by the heating effect. The result is consistent with experimental findings. This result can be applied to study terahertz nonlinear superconducting metamaterials.