Streptococcus pneumoniae (SP) is a key pathogen that induces various infectious diseases such as pneumonia, otitis media, and meningitis. As a core immunogen, its capsular polysaccharide serves as the key target for the research and development of pneumococcal vaccines. The chemical synthesis of structurally well-defined and homogeneous oligosaccharides plays a pivotal role in elucidating the structure-activity relationship underlying the immunological activity of polysaccharides and in the development of novel glycoconjugate vaccines. This study achieved the efficient chemical synthesis of two trisaccharide intermediates of the capsular polysaccharide from Streptococcus pneumoniae serotype 9N. Starting from readily available D-mannose and D-glucose, a reducing-end to non-reducing-end synthetic strategy was adopted. By optimizing key reaction parameters including protecting group screening and solvent effect regulation, two types of 1,2-cis glycosidic linkages were efficiently constructed. Specifically, the yield of the 1,2-cis-α-glycosidic bond reached 90%, whereas that of the 1,2-cis-β-glycosidic bond was tripled, with its stereoselectivity significantly improved. Furthermore, the issue of low nucleophilicity at the C4 position of amino sugars was resolved by the introduction of electron-donating protecting groups at the hydroxy groups adjacent to the receptor glycosylation sites. Eventually, two target trisaccharide intermediates were efficiently synthesized in 11 and 10 total steps, respectively, laying a solid foundation for the preparation of the pentasaccharide repeating unit and longer-chain oligosaccharides of the Streptococcus pneumoniae serotype 9N capsular polysaccharide.