The hydrophobically assisted switching phase (HASP) method was applied in the assembly of oligomannosides. A new mannosyl donor with high reactivity was selected after a series of optimization studies, which was suitable for the synthesis of oligomannosides via the HASP method. The practicability of the HASP method towards the synthesis of branched oligosaccharides was explored and two branched penta-mannosides were assembled efficiently.