<p>Japanese–Chinese bilingual children are rapidly increasing in Japan. Recent research has suggested that bilingual children have a larger vocabulary in their environmental language than their home language. However, Japanese nursery teachers and public therapists have not focused on the children’s home language development. Moreover, using monolingual screening tests for bilingual children leads to under-diagnosing language delays and developmental problems. Nevertheless, only a few Japanese assessment instruments are translated into or from Chinese. We utilized bilingual assessment tools and investigated developmental concerns and language development in 92 Japanese–Chinese bilingual preschoolers aged 1–6 years at risk for autism spectrum disorder (ASD). We found that typically developing children demonstrate increased vocabulary acquisition in both languages by age four. In contrast, children at risk for ASD exhibited gradual vocabulary growth in Japanese but showed no significant development in Chinese comprehensive or productive vocabulary after 30 months of age. Moreover, logistic regression analysis indicated a heightened risk of underdiagnosis among bilingual children with ASD when home language criteria were not considered. These findings underscore the need to employ bilingual screening tools for early identification, particularly around 2.5 years of age, to prevent overlooking delays in bilingual language development.</p>