Abstract: 【Objective】To study population dynamics of Bemisia tabaci and its dominant natural enemies on cucumber and tomato cultivated under greenhouse planting, and to provide scientific basis for comprehensive management of B.tabaci on cucumber and tomato cultivated under greenhouse planting.【Method】Through the systematic investigation, recording the species and quantity of B.tabaci and its natural enemies on cucumber and tomato cultivated under greenhouse planting by five sampling points;the quartile methodwas used to divide its main activity period, and the temporal niche relationships between B.tabaci and its dominant natural enemies were analyzed by using niche index.【Result】The result showed that the dominant natural economies of B.tabaci on cucumber and tomato cultivated under greenhouse planting were Nesidiocoris tenuis, Erigonidium graminicola and Encarsia formosa.The population dynamics of B.tabaci and its dominant natural enemies were slightly different on two protected crop fields.The activity period of B.tabaci on cucumber field was earlier than that on tomato field, and the population growth was quickly.During the main activity period of B.tabaci, the population quantity on tomato field was 1.03 ind/cm², which was significantly higher than that oncucumber field(P< 0.05, the same below).The main activity period of B.tabaci on cucumber field was from seedling period to flowering and fruit setting period, and on tomato field was from tomato early blooming period to young fruit period.There was no significant difference in the population quantity of the N.tenuis and E.graminicola in the two crops fields(P> 0.05).However, the time niche width values of the two natural enemies differred greatly in different planting environments.The niche breadth of E.formosa was smaller and the activity period was more concentrated than N.tenuis and E.graminicola.At the main and late activity periods, the population quantity of E.formosa on tomato field were 0.34 and 0.24 ind/cm², both were significantly higher than that on cucumber field.The largest niche breadth parameter with B.tabaci was N.tenuis on two crops fields, being 0.9664 and 0.9637 respectively.【Conclusion】The main activity period of B.tabaci on tomato field is from tomato early blooming period to young fruit period, and on cucumber field is from seedling period to flowering and fruit setting period.During the main occurrence period of B.Tabaci, chemical control and other control methods should be supplemented to coordinate controlling the population of B.tabaci.