This paper proposes a novel resonant cavity enhanced (RCE) photodetector with a flat-top and steep-edge response, which is promising for WDM application. The simulated response curve has a bandwidth of 1.8 nm at 0.5 dB down from the peak, 5.6 nm at 10 dB down from the peak, 10.4 nm at 20 dB down from the peak, and a peak quantum efficiency of 99.7%, the ripple in the flat-top is negligible.