All-inorganic　halide　perovskite　(IHP)　has　been　deemed　promising　in　photocatalysis　due　to　tunable　bandgap　and　long　lifetime　of　charge　carriers.　However,　unsatisfactory　photocatalytic　activity　and　low　stability　prevent　its　practical　applications.　Rational　construction　of　heterojunctions　has　been　proved　to　be　an　efficient　way　to　circumvent　these　obstacles.　Herein,　g-C3N4　nanosheet　was　employed　to　construct　a　2D/2D　(2D:　two-dimensional)　heterostructure　with　Cs3Bi2Br9　through　an　electrostatic　self-assembly　process.　Owing　to　the　efficient　transfer　of　photogenerated　charge　carriers,　the　activity　of　Cs3Bi2Br9　was　boosted　with　enhanced　generation　of　carbon　centered　radicals.　The　optimized　10%　Cs3Bi2Br9/g-C3N4　composite　displays　the　highest　benzaldehyde　formation　rate　of　4.53　mmol.h(-1).g(-1)　under　visible　light,　which　is　41.8　and　2.3　times　that　of　individual　g-C3N4　and　Cs3Bi2Br9,　respectively.　The　stability　of　Cs3Bi2Br9　nanosheets　and　its　selectivity　for　benzaldehyde　(from　65%　of　Cs3Bi2Br9　to　90%　of　the　composite)　was　enhanced　by　reducing　its　surface　energy　and　tuning　the　reaction　pathway,　respectively.