This study intends to shed light on the inconclusive argument pertaining to children's acquisition of logical form (LF) operation. Specifically, we examined children's interpretations of sentences with the ambiguous modal verb "yinggai" "should," like "Xiaohua yinggai shangchuang shuijiao le", whose meanings depend on the landing sites of "yinggai" at LF ("root interpretation": Xiaohua is obligated to go to bed now. "epistemic interpretation": It is the case that Xiaohua has gone to bed.). The results of truth value judgment task from 15 children (range: 4;8-6;2, mean: 5;4) and 37 adults indicate that both groups tend to interpret the ambiguous "yinggai" as epistemic readings and that children's interpretation is adult-like. Thus, this study supports (Syrett and Lidz's in "Lang Acquis" 16:67-81, 2009) view that 5-year-olds have adult-like LF development and their difficulties in interpreting covert movements may be reduced to extra-grammatical factors.