Abstract: Sheet electron beams possess characteristics including the ability to carry larger currents, easily matching with planar high-frequency circuits, and large heat exchange areas. Hence, developing sheet beam devices is an important way to realize high-frequency, high-power output at the millimeter-wave band. To further enhance the overall efficiency of sheet beam klystrons, the depressed collector must be introduced to recover the residual energy of electrons after interaction, while also helping to lower the heat dissipation on the collector surface. Current research on depressed collector technology primarily focuses on the round electron beam. This paper investigates the recovery efficiency of depressed collectors for the W-band sheet beam klystron, concretely, that involves analyzing the energy and velocity distribution of the spent electron beam, designing a single-stage depressed collector to verify initial feasibility, and progressively optimizing stages and structure. Using a typical beam state after interaction as the input condition, the calculation result indicates that, in the absence of electron backstreaming, the recovery efficiency of the single-stage depressed collector can reach 53.5%, and that of the optimized three-stage depressed collector can reach 68.0%, increasing the overall efficiency of the sheet beam klystron from 12.5% to 28.8%.