The structure and size of bimetallic catalysts play a crucial role in many important chemical transformations. Controlled synthesis of bimetallic nanoparticles avoiding overgrowth and aggregation can be achieved by surfactants, which are always detrimental to catalytic performances and understanding of structure-property relationship. Preparation of surface-clean bimetallic catalysts with uniform size and well-defined structure is still challenging. Herein the Au-Pt bimetallic nanoparticles immobilized on SBA-15 were prepared by facile adsorption-reduction method. Characterizations showed that Au-Pt bimetallic nanoparticles were evenly confined within the mesopores of SBA-15, possessing the uniform size of 6.0 nm and existing in the form of alloy structure. For the first time the Au-Pt bimetallic nanoalloys with Au-to-Pt molar ratio of 5:1 (Au₅Pt₁@SBA-15) exhibited a lot higher activity than monometallic Au@SBA-15 and Pt@SBA-15 catalysts with excellent selectivity towards 1,2,3,4-tetrahydroquinoline for chemoselective hydrogenation of quinoline in water under very mild conditions. It was superior to other heterogeneous catalysts reported to date. The observed properties were related to the synergic effect between Au and Pt. The Pt sites of high electron density originated from the electron transfer between Au and Pt enhanced the ability of H₂ dissociation and then provided a new and main approach to H₂ splitting, while the Au sites accounted for the adsorption and activation of quinoline. This precise synthetic strategy will be very helpful to explore surface-clean gold-based bimetallic catalysts with high performance for selective hydrogenations.