Transparent gamma-Cu I crystal with size about 9 mm x 9 mm x 1 mm was grown by using optimized solvent evaporation method with gamma-Cu I powders as raw material and acetonitrile as solvent. The results of Rutherford backscattering spectrometry (RBS) showed that the proportions of copper and iodine in as-grown gamma-Cu I crystal were 53.1 and 46.9 mol%, respectively. By iodine doping into growth solution, the proportions of copper and iodine were adjusted to 50.4 and 49.6 mol%, respectively. Photoluminescence (PL) showed that the as-grown gamma-Cu I crystal had an emission at 410 nm and an emission band around 720 nm. After iodine doping, the emission band around 720 nm was suppressed while other two weak emissions at 424 and 474 nm appeared. X-ray excited luminescence (XEL) showed that the as-grown gamma-Cu I crystal had an emission at 435 nm and an emission band around 720 nm. After iodine doping, the emission band around 720 nm was suppressed. Comparing the emission spectra before and after iodine doping, it is clear that red luminescence in gamma-Cu I crystal could be suppressed by iodine doping. The effect originated from the reduction of iodine vacancy in the crystal, but not from the iodine adsorbtion on the surface.