Urban parks are essential to maintaining healthy, livable, and sustainable cities. It is vital to ensure urban parks serve the communities equally, particularly during the pandemic and for traditionally disadvantaged groups. This study examined how the COVID-19 pandemic changed urban dwellers' visitation patterns to urban parks and assessed whether the pandemic exacerbated the inequalities of urban park access among different sociodemographic groups in the U.S. We investigated urban dwellers' visitation patterns from two aspects (origin and destination) and used a big data approach by integrating mobility data from mobile devices, remote sensing/geospatial data, socioeconomic data, etc. This study found urban dwellers preferred visiting urban parks that were remoter, larger, greener, less crowded, less noisy, and with less human activities during the pandemic (i.e., 2020 and 2021) vs. pre-pandemic (i.e., 2019). Our results also highlighted that the pandemic exacerbated social inequalities in urban park access. The urban dwellers from block groups with higher percentages of racial/ethnic minorities, higher unemployment and poverty rates, and lower income were more negatively influenced by the pandemic and became less likely to visit urban parks. More importantly, such social inequalities further increased in 2021 compared with the early pandemic period of 2020.