The quasi-elliptic multi-mode bandpass cavity filters and duplexer with slot mixed-coupling structure are proposed in this paper. A single metal cavity embedded with a rectangular- shaped slot-cut metal plate is utilized to constitute a multi-mode bandpass filter with a few features including wide passband, low profile and controllable transmission zeros (TZs). In this paper, the slot-cut metal plane serves as the multi-mode resonators. In detail, the slot on the metal plane functions as the circuit element to move the higher order modes within the reasonable frequency-band, while serving as a mixed-coupling structure to generate out-of-band transmission zeros. To demonstrate the multiple-mode capability in filter design, the dual-mode, triple-mode, and quadruple-mode filters are developed by appropriately allocating a few TZs in the upper stopbands using the proposed slot mixed-coupling approach, namely, Type-I filters. Next, a quadruple-mode cavity filter with both lower and higher TZs is designed, namely, Type-II filter, which is further applied for the exploration of a duplexer. Finally, the filter and duplexer prototypes are fabricated and measured. The measurement results are found in good agreement with the simulated ones.