Parents of children with ASD experience a higher incidence of mental health difficulties, including stress, depression, and anxiety, than parents of children without ASD. According to studies related to ASD, parent–child physical activity programs are an effective approach to encourage both parents and their children with ASD to exercise together, thus improving the mental health of parents due to this interactive family activity. The purpose of the present study was to explore the effects of this web-based parent–child physical activity program on the mental health of parents of children with ASD. A total of 94 parent–child pairs consented to participate in this study, and 75 parent–child pairs completed the study. Three instruments—DASS-21, PSI-4-SF, and WHOQOL-26—were used to measure mental health, parental stress, and quality of life, respectively. A randomized controlled trial design was implemented to examine the effectiveness of the 10-week web-based parent–child physical activity program on improving the mental health of parents of children with ASD. The results showed that after the 10-week parent–child physical activity program, there were significant differences in overall DASS-21 and PSI-4-SF for the experimental group, compared with control group (p < 0.05), which indicated that the parent–child physical activity program has a positive influence on mental health in parents of children with ASD. One sub-area of WHOQOL-26 between the experimental and control groups across pre-/post-testing intervals also showed greater reductions in the item of psychological health (p < 0.05). In conclusion, the findings demonstrated the efficacy of the web-based parent–child physical activity program for improving mental health in parents of children with ASD.