The interaction of an intense femtosecond laser with gas to produce high-order harmonics is an important ultrafast coherent light source. Our simulation shows that a mid-infrared femtosecond laser pulse can enhance the multi-photon resonance between the ground states and excited states of the atom by the ac Stark effect and generate high-brightness monochromatic high-order harmonic radiation. By solving the time-dependent Schr dinger equation, we obtain unusual resonantly enhanced high-order harmonics below the threshold, the intensity of which is strongest at an optimal laser intensity. Further, the time-frequency analysis shows that the resonance enhancement is related to the second-order ac Stark effect in a high field, while insensitive to the laser wavelength. This new mechanism makes mid-infrared femtosecond laser pulses more conducive to the generation of ultra-fast monochrome ultraviolet/extreme ultraviolet light sources with high brightness and has important application prospects in condensed matter physics and materials science.