By solving the time-dependent Schrödinger equation under velocity gauge with split-operator method, we studied the high harmonics of one-dimensional helium atom exposed to one-color laser field and two-color fields composed of IR field and UV field. And the relationship between conversion efficiency of high-order harmonic generation near the cut-off position and wavelength of the laser (800—2000 nm) was also analyzed. It was found that high-order harmonic conversion efficiency driven by two-color laser field near the cut-off position changes with wavelength as η(λ)∝λ－x,where the x value depends on the intensity of laser field, and the appropriate selection of intensity of combined laser field can improve high harmonic conversion efficiency near the cut-off position.