Teacher talk has been found to significantly affect children's early language development during shared book reading; however, the mechanism of such influence remains unclear. The current study has zoomed in on vocabulary development in Mandarin as a heritage language (HL) in Singapore kindergartens, 1) exploring the linguistic features (i.e. lexical diversity and syntactic complexity) of these teachers; and 2) investigating the mediation effects of the significant linguistic feature(s) in teachers' qualifications (i.e. degree, experience, and language proficiency) and children's Mandarin vocabulary growth. 31 Mandarin teachers and 441 English-Mandarin bilingual children (4-5 years old) were investigated during their shared book reading sessions. Teacher talk was transcribed with CHAT and two indicators of language complexity (D and Mean Length of Utterance in word) were calculated with CLAN. The mediation effects of teachers' language complexity were explored using a path model. Results showed that after controlling for the children's initial vocabulary, maternal education, nonverbal intelligence, and gender, teachers who used more complex syntax were found to be associated with children's higher growth in Mandarin receptive vocabulary over a school year. Teachers' qualifications (i.e. experience and Mandarin proficiency) had no direct influence on children's language growth, but demonstrate their impact via teachers' syntactic complexity.