Studies of language development in children with autism spectrum disorder (ASD) have been primarily focused on the influence of child-based factors such as autism traits, IQ, and initial language skills. Yet the findings of these studies are inconclusive. There has, moreover, been little research compared the relative influences of child-based factors with environmental factors, (e.g. parental inputs). The current study attempts to fill this research gap by examining a range of both child-based factors and parental inputs. We measured the structural language abilities manifested in parent-child interactions over four time points across nine months in 42 Chinese-speaking autistic children (M = 57.42 months, SD = 11.39). Our results showed that children's mean length of utterance (MLU), word types, and word tokens grew rapidly, but their development trajectories varied. Initial expressive language ability was a significant predictor of children's language outcomes, while nonverbal IQ and autism traits did not relate to children's language abilities when controlling for initial expressive language ability. Parents' MLU, word tokens, and word types did not associate with children's structural language abilities. The findings shed lights on the importance of one of the child-based factors in particular, that is, initial expressive language skills, in the language development of autistic children.