The　photocatalyst　of　TiO2/SnO2　compound　semiconductor　was　prepared　by　improved　sol-gel　method.　The　coupling　mechanism　and　the　factors　affecting　photocatalytic　efficiency　were　studied　by　using　XRD,　gas　chromatography,　granularity　equipment　and　surface　photovoltage　spectroscopy.　TiO2/SnO2　catalysts　were　employed　for　photocatalytic　degradation　of　formaldehyde,　The　experimental　results　showed　that　phtocatalytic　efficiency　of　TiO2　catalyst　was　greatly　enhanced　by　adding　SnO2　into　TiO2,　and　the　optimum　content　of　SnO2　was　about　20　%　(mol),　According　to　the　particle　close　packing　rule　and　experimental　result,　the　strong　and　weak　coupling　effect　models　for　TiO2/SnO2　compound　semiconductor　were　proposed.　The　coupling　effect　was　derived　from　the　close　packing　structure　of　the　photocatalyst　particles.　The　critical　bourn　between　enhanced　and　weaken　effect　depends　on　whether　the　close　packing　structure　is　three-dimensional　or　not.　The　photocatalyst　doped　with　SnO2　less　than　20　%　(mol)　presented　a　three-dimensional　structure,　which　improved　the　activity　of　the　photocatalyst　due　to　effective　separation　of　the　photogenerated　carriers,　The　opposite　situation　was　occurred　when　content　of　SnO2　was　more　than　20　%　(mol).