Parents' math anxiety may hinder their engagement in math activities with their children. This study investigated when this impact occured by examining the association between parental math anxiety and the frequency of home-based math activities among parents of preschool-aged children, and investigating whether this parental math anxiety-activity relation was moderated by age of children and other sociodemographic factors. A total of 648 Chinese parents of preschool-aged children reported their math anxiety and the frequency of math activities at home through online surveys. Their frequency of math activities increased with some sociodemographic factors, including parental educational level, the number of household books, and children's age. Critically, the frequency of math activities overall decreased with increasing parental math anxiety, and this relation was moderated by children's age: the frequency of math activities significantly decreased with parental math anxiety when their children were older than age of 4.5 years (4y6m - 6y0m), but this relation was not significant when children were younger (3y0m - 4y6m). No other sociodemographic factors moderated the parental math anxiety-activity relation. These findings suggest that the adverse effect of parental math anxiety manifests even before children enter elementary school, and highlight the necessity for early interventions to address parental math anxiety.