In Mandarin, “yao” has more than one meaning. It can be used as a regular verb which means “to want”; it can also be used as a future tense indicator which means “will” or “shall”. The author's hypothesis in this paper is that when “yao” is used in sentences to indicate future tense, the predicates of such sentences are serial verb constructions; “yao” functions as bleached light verb to indicate future tense. Empirical approach was used to test the hypothesis. The subjects of the experiment were two Mandarin native speakers. 40 sentences, including 8 distracters, were given to the subjects at random, and PRAAT software was used to record their speaking. Results show that when “yao” is used as a regular verb, it gets higher pitch; in this situation the difference of pitches between “yao” and its previous morpheme or the latter morpheme is much more significant. Therefore the conclusion is that when “yao” is used in sentences to indicate future tense, the predicates of such sentences are serial verb constructions; “yao” functions as bleached light verb to indicate future tense.