Abstract: Based on a systematic field investigation, the methods of the fuzzy cluster analysis and the relationship analysis of temporal-dynamic spatial niche breadth with population density were used to study the population dynamics of Plutella xylostella (L.) in spring cabbage fields in Hefei, in order to provide basis for scientific management of diamondback moth. The research results showed that the population dynamics of Plutella xylostella (L.) in spring cabbage fields could be divided into three development periods: dot and patch occurrence period (low population density and high patchiness), expansive occurrence period (medium population density and low patchiness) and rampant occurrence period (high population density and low patchiness). The damage of diamondback moth to spring cabbage was quite complicated. Diamondback moth attacked the heart leaves of cabbage mainly in low population density and dispersed gradually to inner and outer leaves of cabbage with the growth of cabbage and the rise of its population density and led to the damage of whole cabbage plant eventually. The dot and patch occurrence period of diamondback moth population in spring cabbage fields was the critical control time.