By optimizing the phase matching condition of high harmonic generation (HHG) from a supersonic neon gas jet, the enhanced HHG in the region of 60?70 eV has been selected. Three-dimensional numerical calculation shows that plasma plays a significant role in the phase matching process of HHG in a supersonic gas jet with short medium length. Due to plasma formation, the harmonic emission decays as the laser intensity reaches over 3.5 ￡ 1014 W/cm2. The plasma induces the broadening and blue shift of the HHG spectra, which provides a method for fine-tuning the harmonic wavelength.