High-order optical harmonic emission by a single atom exposed in a partially coherent intense laser field is numerically calculated. We found that all harmonic shift towards high-frequency and the harmonic spectrum is broadened in different degree according to different harmonic order. Comparing with that of monochromatic field atoms are ionized easier by the partially coherent field. It is possible that the wavelength of the harmonic can be tuned by adjusting the coherence of the incident laser field.