In the paper the transmissions of X-ray through pure gold material and through copper-dopped gold material are studied by MCNP simulations. The results show that, the transmission in pure gold is significantly weaker than that in copper-dopped gold. A new approach to detect the purity of gold is proposed accordingly. To get a better detection, one may choose X-ray with emission energy 0.3 MeV and detection spectrum 0.11-0.16 MeV. The approach is easy, cheap and feasible, furthermore, it also has advantage of wide-detection-range. The study provides guidance to detect purity of gold material.