This study investigated the generation of unconventional language in the spontaneous speech of Chinese adults with autism spectrum disorder (ASD), and how it was related to their grammatical performance, when compared to neurotypical (NT) controls. Twenty Cantonese-speaking adults with ASD and 20 NT controls completed three interview tasks in the Autism Diagnostic Observation Schedule, Second Edition (ADOS-2), and their spontaneous speech was recorded and transcribed. Utterances containing unconventional language (neologisms, idiosyncratic phrases, and pedantic language), morphosyntactic errors, mean length of utterance (MLU), and mazes were computed. The ASD group produced more neologisms, idiosyncratic phrases, and pedantic language than the NT group and their grammatical difficulties were shown in shorter MLU but not morphosyntactic errors. Mazes were more frequent in the ASD than the NT group. While the use of unconventional language increased with MLU in the NT group, it correlated positively with mazes in the ASD group. Generation of unconventional language, particularly pedantic language, in Cantonese-speaking NT adults is linked to more advanced grammar, while it appears to be a common speech characteristic among autistic speakers regardless of individual grammatical performance.