This　paper　addresses　the　cooperative　control　problem　of　vehicular　platoons　on　a　loop　road.　By　integrating　speed-density　relationship　into　the　control　strategy　design,　a　novel　distributed　control　protocol　is　proposed.　The　cooperative　control　problem　is　reformulated　as　a　multi-agent　consensus　problem,　which　is　further　transformed　into　a　partial　stability　problem　through　linear　transformation.　Using　partial　stability　theory　and　the　auxiliary　system　method,　we　derive　a　necessary　and　sufficient　condition　for　solving　the　cooperative　control　problem.　Finally,　numerical　simulations　are　conducted　to　verify　the　effectiveness　of　the　proposed　control　scheme.　©　Beijing　Paike　Culture　Commu.　Co.,　Ltd.　2025.