Abstract:The sequence of Mandarin third tone sandhi above the phrase level always takes flexible forms.It turns out to be correlated with not only phonetic contexts, but also various factors including syntax and prosodic structure.It has also been widely studied in the field of linguistics (Cheng 1973; Chao 1968;Chen 2000).The five-syllable phrase Lao Li Mai Hao Jiuis a typical case studied by various schools and different principles are proposed, e.g.Syntactic Boundary Depth, Branch Condition and Foot Formation Rules.Based on previous works, the present article deals with this case from the perspective of syntax-prosody interface and within the framework of Optimality Theory, accommodating End-based approach (Selkirk 1986, 1995, 2009) to the study of tone sandhi domain.It is held that this kind of issue could be well accounted for via adjustment of the ranking hierarchy of interface related constrains.