By numerically solving the time-dependent Schrödinger equation with the interaction between one-dimensional multi-well potential and the mid-infrared few-cycle femtosecond pulses, we theoretically investigate the high-order harmonic generation spectra in a crystal, further find the cutoff frequency formula under the new condition. Our results clearly show that the high order harmonic generation in the crystal is fundamentally different from that in the atomic case, owing to the high density and periodic structure. The harmonics spectrum shows a cutoff position that scales linearly with the peak amplitude of electric field of the drive laser and the lattice parameters. Based on the important role of the three-step model obtained by quasi-calssical mechanics method in gas harmonic generation, in this paper, this method is also well used to verify the cutoff position law in crystal harmonic generation.