High speed and long wavelength photodetector is the key device for high-bit-rate optical fiber communication systems and optical networks that need wide frequency bandwidth and high external quantum efficiency simultaneously. It is well known that to increase the speed of PIN photodetector, the absorbing layer thickness should be reduced. The reduction of the absorbing layer thickness means a reduction in the quantum efficiency. So for conventional PIN photodetectors, there is a tradeoff between the quantum efficiency and the device frequency bandwidth through the absorbing layer thickness. One effective solution to this conflict is to use a resonant cavity enhanced (RCE) photodetector. In the RCE photodetector, the absorbing layer is put inside a Fabry-Perot cavity. This means a very thin absorbing layer thickness can be used to achieve a high quantum efficiency and wide frequency bandwidth. In this paper, analyses of light field distribution in the absorption layer of RCE photodetector based on practical design and manufacture are performed. The expression of light field distribution is used in the continuity equations to the frequency response of the RCE photodetector. The frequency response of long wavelength RCE photodetector is analyzed and calculated. The result of analyses and calculation is almost identical with the measuring result of the wide frequency bandwidth RCE photodetector manufactured. That device is an InP-based long wavelength resonant cavity enhanced photodetector with InP-Air gap reflectors and has 8 GHz frequency bandwidth and about 60% external quantum efficiency with the active area of 50 μm×50 μm.