Quantum interference in population trapping in a multi-level system,which is coupled by vacuum modes and driven by a coherent field,is studied in view of dressed states.The analytic expression of trapping population under interference conditions is derived.The influences on population trapping of the initial population,the Rabi frequency of the driving field and the spacing between the upper levels are explored.It is shown that the forms of the total trapping population of all energy-levels under certain initial population condition,depending on both the Rabi Frequency of the driving field and the spacing between the upper levels,are not always the monotonically increasing (or decreasing) functions.The maximum of the total trapping population in a given system can be obtained by calculating the extremal solution of its analytic equations.