Expressive incoherence can be implicated in socio-emotional communicative problems in autism spectrum disorder (ASD). The present study examined expressive incoherence in 37 children with ASD and 41 typically developing (TD) children aged 3–13 years old during a frustration task. The role of alexithymia in expressive incoherence was also assessed. Compared to TD children, children with ASD had higher expressive incoherence, such as more neutral and positive emotion expressions during negative behaviors, but not in the expression of negative emotions during positive behaviors. Further analyses revealed that alexithymia moderated the expressions of positive emotions during negative behaviors. These results suggest that children with ASD may benefit from interventions targeting alexithymia to increase emotional coherence, which may improve socio-emotional communication.