To analyze the causes of weed community diversity and their strategies of adaption to the high heterogeneity of urban habitats, weed communities in the central urban area of Harbin, China were studied, and a classification system was established for the weed communities. There were 175 weed species, belonging to 128 genera and 38 families. The heterogeneous urban habitats and species' temporal niche differentiation resulted in the highly diversified weed communities. The high proportions of mono-species dominance and annual species dominance communities were their response to the unstable urban habitats under human disturbances with high intensities and frequencies. A four-level classification system was established in terms of plant species and habitat conditions. Within this system, the identified 1763 weed communities could be categorized into two types of life form, 5 types of dormancy form, 22 community groups, and 119 dominance communities.