Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social communication, repetitivebehavior and restricted interests. Balance ability, as a crucial domain of gross motor capacity, holds significant importance in assessing and intervening the developmental level of children with autism, particularly deserving attention. However, current assessment methods heavily rely on subjective observations and qualitative assessments, which makes it challenging to accurately and objectively measure the balance abilities of children. To address these challenges, we design a balance assessment system for children with ASD, which includes a serious game protocol to guide children in making corresponding balance movements, and an online action detection algorithm specifically designed for balance movements toprovide objective evaluation. Preliminary experiments were conducted on ten adults with typical neurological systems to validate the feasibility and accuracy of the system. The research findings demonstrate the potential of the system to support assessments and further treatment and intervention for children with ASD.