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Background This study aimed to investigate the associations between preconception and prenatal smoking and alcohol consumption and the risk of autism-like behaviour in preschool children, with the aim of providing evidence for early prevention strategies.Methods This cross-sectional study included 19,089 children aged 3–6 years from 111 kindergartens in Huicheng District, Huizhou City, Guangdong Province, China, between June 2022 and June 2023. Logistic regression was used to analyse the associations between parental smoking, drinking and autism-like behaviour in children. Multiplicative and additive interaction models were applied to evaluate the interaction between smoking and drinking on autism-like behaviour in children.Results Compared with the unexposed group, children with maternal prenatal passive smoking exposure (OR = 1.327, 95% CI: 1.042 to 1.690) and those with a paternal history of smoking (OR = 1.643, 95% CI: 1.036 to 2.605) exhibited significantly higher odds of autism-like behaviour. Among mothers with a prepregnancy smoking history, a significant negative additive interaction was observed between smoking cessation during pregnancy and concurrent alcohol consumption (RERI = −2.170, 95% CI: −4.035 to −0.304).Conclusions This study identifies prenatal exposure to parental smoking—both maternal passive smoking and paternal smoking history—as a significant risk factor for autism-like behaviour in preschoolers. The finding of a negative additive interaction between maternal smoking cessation and concurrent alcohol consumption underscores the complexity of modifying health behaviours. These findings emphasise the key role of reducing parental tobacco and alcohol exposure in the primary prevention of autism spectrum disorder.