The mathematical abilities of children with autism spectrum condition have been understudied. Magnitude representation is a fundamental numerical ability that emerges early in development and is linked to children's learning of formal mathematics. It remains unclear about whether children with autism spectrum condition differ from their peers without autism spectrum condition in the precision of magnitude representations. This study recruited preschool-aged children with autism spectrum condition (N = 70; 64 boys, M[subscript age] = 5.20 years) and without autism spectrum condition (N = 117; 63 boys, M[subscript age] = 5.11 years), and tested their precision of magnitude representation with an approximate number comparison task (dot comparison). Children with autism spectrum condition exhibited the lower numerical comparison accuracy (i.e. the weaker magnitude representation) than their peers without autism spectrum condition, regardless of the congruency between numerosity and surface area of dots. Moreover, the lower numerical comparison accuracy was observed even controlling for multiple general cognitive abilities (working memory, inhibitory control, and nonverbal intelligence) and language abilities. In addition, the variability of the comparison accuracy was larger in children with autism spectrum condition than without autism spectrum condition. These findings suggest that children with autism spectrum condition are at risk of weaker magnitude representation from an early age, emphasizing the need for specialized mathematics education or interventions to support their learning.