The planted vegetation in urban green spaces results in high consumption of resources and landscape homogeneity. With the increasing ecological awareness, spontaneous plants have gained attention at home and abroad for their variety of positive attributes, including self-reproduction, low-maintenance, and ecological benefits; however, previous studies have primarily used the city environment as the study area. The distribution of spontaneous plants in urban green spaces has rarely been reported. To determine the diversity, as well as the temporal and spatial patterns of spontaneous plants in urban green spaces, we used the Beijing Olympic Forest Park as the study area for this research project. A total of 123 sample sites were selected using a 200 m grid placed over the park. Each site was a 20 m×20 m square, and all sample sites were surveyed seven times during the growing season from March to November, 2015. Combined with diversity, community cluster calculation, and Duncan tests, we analyzed the composition features, diversity, and community classification of spontaneous plants in this urban park. A total of 128 plant species were recorded, which belonged to 98 genera and 32 families. Among them, native plants accounted for 76.56% and alien species accounted for 23.44%, meanwhile, invasive species accounted for 12.50%. The species number of spontaneous plants exhibited a pattern with a single peak during the year, and the peak occurred in late August with 98 species. The number of species in the family Compositae was significantly higher than that in other families, and the percentage of foreign species increased in summer. The habitat of roadside and woodland contained the highest number of sample sites as well as more spontaneous species, 109 and 106, respectively. Community diversity was significantly higher in early October, followed by late August, late April, and early June. Community diversity among different habitats was not stable because of the varying degrees of external interference in different seasons. All communities were classified into 42 community groups based on species dominance. Each season and habitat had its own dominant species and communities, and nine community groups occurred in all the three seasons. With the variation among seasons, most groups presented a rich and colorful effect together with that of wildness. The results of this research will provide a reference for the future construction of sustainable urban vegetation with low maintenance, high biodiversity, and regional characteristics.