Compared to official sports mega events, grassroots sports events are attractive to participants because of their universality, accessibility, and casual nature. Taking the Shenzhen 100 km Hikathon as an example, this study investigates the effect of grassroots sports events on sustainable development in migrant cities through residents’ perceptions of such events, and how these affect support. We collected 59 questionnaires in a pre-survey and 612 surveys for formal analysis, and used SPSS and AMOS software to construct a structural equation model. The results indicate that the Hikathon’s popularity, low media impact, small scale of investment and construction, and short duration had fewer negative effects and was beneficial to sustainable development for the migrant city. Residents perceived more positive benefits (improved city image and economic, environmental and cultural benefits) and less negative costs (environmental and traffic costs), which lead to broader support for such events. Among residents’ sociodemographic characteristics, only age was found to moderate the relationship between perceived effects and support. The findings suggest that residents generally perceive grassroots sports events positively, especially in migrant cities, such as Shenzhen, where community events are considered to serve an important role in the construction of place identity.