This study aimed to determine whether there is a correlation between maternal conversational participation and the unconventional language of autistic children, and whether the relative vocabulary diversity between mother and child would affect the relationship between them. Participants were 39 autistic Mandarin-speaking children, aged 3-6 years, and their mothers. The mother-child interactions were video-recorded; then, all interactions of the conversations were corpus coded and subsequently analysed. Mother's mean length of turn (MLT) was used as a measure of maternal conversational participation. The ratio of mother to child word types (Types[subscript m/c]) measures the mother-child relative vocabulary diversity. The results indicated a significant negative correlation between maternal conversational participation and the unconventional language of autistic children, and mother-child relative vocabulary diversity moderated the relationship between maternal conversational participation and the unconventional language of autistic children. We provide two specific excerpts of play situations showing the interaction of mothers with similar MLT but different Types[subscript m/c] (Excerpt 1: High Types[subscript m/c], Excerpt 2: Low Types[subscript m/c]). The results provide insights for further research into the relationship between maternal input and autistic children's language development.