Abstract: Halliday (1975, 2007) divided child language development into three phases: protolanguage, transition and adult language and analyzed the criteria of entry into Phase I and Phase Ⅱ and the functions performed and meanings realized by conducting a case study. Talking about when a child can be said to have entered Phase Ⅲ, Halliday (1975) held: “By the end of Phase Ⅱ, the child has entered the adult language. He has built up a system that is multi-stratal (content, form and expression) and multifunctional (ideational, interpersonal and textual). From this point on, he is adding to what he already has. He has learned how to mean; his language development now consists in extending his range of meaning potential to broader cultural horizons.” Halliday (2007) took dialogue as another important criterion. In order to get a better understanding of the law of child language development, this paper attempts to prove through a case study that interpersonal metaphor, structural completeness and texture should be regarded as the criteria together with multistratification, multifunctionality and dialogicity to judge whether the child has entered Phase Ⅲ.