The　higher　contact　stiffness　of　the　double-locking　toolholder-spindle　(BTF40-type)　joint　is　helpful　in　improving　the　machining　accuracy　and　cutting　stability　of　high-speed　machining　center.　In　this　article,　an　optimization　technique　is　introduced　to　obtain　the　high　contact　stiffness　of　BTF40　toolholder-spindle　joint　at　the　high　speed.　The　torsional　and　radial　stiffness　at　25,000r/min　can　be　obtained　by　using　the　macro-micro　hybrid　method.　The　contact　ratio　of　taper　surface　and　maximum　nodal　pressure　of　contact　surfaces　is　used　as　constraint　conditions　to　ensure　the　connection　reliability　and　the　working　life　of　the　toolholder-spindle　system.　The　particle　swarm　optimization　algorithm　with　suitable　parameters　for　this　problem　is　used　to　search　the　optimal　stiffness　of　joint　automatically.　The　optimal　stiffness　of　joint　and　pressure　distribution　of　taper　surface　are　compared　with　the　initial　values　for　validating　the　effectiveness　of　the　optimization　results.　This　research　is　valuable　for　guiding　the　application　of　BTF-type　toolholder-spindle　system　at　the　high　speed　25,000r/min.