Developing　a　cyanide-free　gold　electrodeposition　system　is　significant　for　replacing　the　cyanide　electroplating　gold　plating　system,　which　is　highly　harmful　to　the　environment　and　toxic　to　human　health.　In　our　study,　a　cyanide-free　system　for　electrodepositing　gold　(Au)　coatings　was　proposed　with　AuCl　as　the　main　salt　and　5,5-dimethyl　hydantoin　(DMH)　as　the　Au　(I)　complexing　agent.　Through　theoretical　calculations　and　electrochemical　test　screening,　4,6-dimethyl-2-mercaptopyrimidine　(DMP)　is　suitable　for　DMH-based　gold　electrodeposition　and　was　selected　as　an　electroplating　additive.　The　role　of　DMP　in　gold　electrodeposition　was　further　studied.　In　particular,　the　adsorption-coordination　mechanism　was　proposed　to　elucidate　the　role　of　DMP,　based　on　calculations　of　the　Bader　charge　density　difference　for　DMP　adsorbed　on　the　gold　surface　and　the　interaction　between　DMP　and　Au+.　Electrochemical　tests　such　as　cyclic　voltammetry　and　chronoamperometry　were　utilized　to　study　the　impact　of　varying　DMP　concentrations　on　gold　electrodeposition　behavior.　Chronoamperometric　analysis　indicates　that　Au　electrodeposition　obeys　a　3D　diffusion-controlled　progressive　nucleation　process,　independent　of　DMP　concentration　and　applied　potential,　while　the　presence　of　DMP　markedly　suppresses　grain　growth.　The　addition　of　DMP　significantly　improved　the　surface　morphology　and　brightness　of　the　gold　coating,　resulting　in　a　smooth　and　dense　layer　with　a　golden-yellow　appearance.　XRD　results　showed　that　the　addition　of　DMP　reduced　the　grain　size　of　the　Au　coating　from　22.1　to　15.7　nm,　with　the　gold　coating　primarily　growing　along　the　Au　(111)　plane.　The　XPS　test　results　show　that　the　purity　of　the　gold　plating　layer　is　high,　and　there　is　no　inclusion　of　additives　in　the　coating.