Natural secondary brush communities in degraded evergreen broadleaved forests may regenerate gradually after firewood harvesting stops. However, external interferences are likely to cause disturbance, especially as a result of brushfires caused by people in surrounding urban areas. A theoretical basis for the successional restoration of natural secondary scrub can be obtained by comparing the responses of community biodiversity to different forest fire intensities. To test whether heterogeneity of forest fire severity inside a stand is related to microenvironmental plant diversity, fire plots were established in a burned secondary shrub area according fire severity and were surveyed after 3 years in a suburb of Taiyang, Linan City. The plots contained 83 species of higher plants, which belonged to 67 genera in 38 families (mostly subtropical), including 21 tree species. The plant community height was less than 2 m and was characterized by deciduous secondary shrub degraded from evergreen broadleaved forest. Low fire severity had no obvious effect on species composition of the shrub layer, but medium fire severity had significant effects. The species composition of the herb layer changed significantly under low and medium fire severity. Fire severity influenced shrub richness and diversity in the following order:low fire severity > unburned > medium fire severity. Richness, diversity, and evenness of the herb was influenced as follows:medium fire severity > low fire severity > unburned. The species composition and diversity of the herb layer was more strongly affected by fire than was the shrub layer. Our results showed obvious spatial heterogeneity of fire severity in the stand, with a mosaic of unburned patches and low and medium fire severity even within a small area. This indicated that spatial heterogeneity of forest fire severity within a section of secondary shrub could cause difference initial diversity responses. Low fire severity would lead to increased plant diversity and community regeneration, but medium fire severity would decrease the diversity of woody plants and increase the diversity of the herb layer, which would inhibit succession from secondary shrub to evergreen broadleaved stand. It is important to cut grasses and remove competing stems around target trees. Although the spatial heterogeneity of forest fire severity in a small area caused spatial heterogeneity in vegetation composition and diversity, it is uncertain whether this heterogeneity will continue to lead to different forest types in the medium-to long-term periods.