Numerical studies are carried out to investigate the effect of the strut axial length on the cold flow field of a trapped vortex combustor(TVC). The results indicate that the flow field in the cavity of a TVC depends to a great extent which exceeds the general expectation upon the axial length of the strut. Two characteristic length Lc1=1 mm and Lc2=9 mm are achieved from detailed numerical simulations of the cold flow field. When the strut axial length is less than Lc1, a great amount of mainstream is entrained into cavity and there exists only a single vortices in the cavity, whereas when the strut axial length is more than Lc1 and less than Lc2, the entrainment of the mainstream into the cavity seems to be eliminated and a dual-vortex dominant structure which consists of two vortices is observed in the cavity. And last, when the strut axial length exceeds Lc2, the flow in the cavity becomes independent of the axial length of the strut. As a result, changing axial length of the strut is regarded as a passive approach to control entrainment of mainstream into the cavity as well as interaction between the mainstream and cavity.