Researches on the effect of bilingualism/diglossia on academic performance conclude disparately yet converge on the children of early school age. This research aims to figure out the longer-term influence of Cantonese usage on academic performance by conducting a large sample survey and the Chinese reading literacy assessment of PISA 2018 on 100 young citizens aged 18 to 25 in Guangdong province. In addition to reconfirming the vastness of Cantonese-Mandarin bilinguals in Guangdong province, China, The Language History Questionnaire 3 detected a categorically strong dominance of Mandarin in the Cantonese districts of Guangdong. Nevertheless, it is still not suggested that the Guangdong parents preclude Cantonese from family language planning out of worry about the childrens academic performance, drawing on the result that no negative effect of Cantonese usage was found in the correlation test.