<p>This study aimed to assess the effectiveness of PMIs for ASD under 3&#xa0;years, and explore potential moderators influencing the effectiveness through moderator analyses. The study searched five English databases for randomized controlled trials (RCTs). The meta-analysis was conducted using a random-effects model to calculate Hedges’s g. Subgroup analyses and meta regression assessed the effects of potential moderators on PMIs effectiveness, with evidence quality evaluated using GRADE. A total of 31 RCTs were included in the systematic review, with 26 included in the meta-analysis. The results showed a small overall beneficial effect of PMIs on ASD under 3 years (<i>g</i> = 0.20). Small to trivial positive effects were found in several subdomains, including adaptive skills (<i>g</i> = 0.29), parent responsiveness (<i>g</i> = 0.23), parent–child interaction (<i>g</i> = 0.35), social communication (<i>g</i> = 0.18), and symptoms (<i>g</i> =  − 0.22). However, PMIs did not show statistically significant effects on children’s cognitive competence, language, or motor skills domains. Subgroup analysis and meta-regression explored potential moderators, but none significantly influenced the effectiveness of PMIs. The GRADE assessment showed that the certainty of the evidence ranged from moderate to low. This study confirmed that PMIs demonstrate positive effects on children under 3&#xa0;years old with ASD, and showed beneficial outcomes in most subdomains. However, the evidence was of moderate to low certainty, so these findings should be interpreted with caution. In the future, broader databases and more large-scale, multicenter, high-quality clinical studies are needed to confirm these effects.</p>