A novel superimposed photodetector (PD) is put forward. The photodetector can obtain a couple of differential photocurrent signals from one input optical signal. The light injection efficiency and the vertical work distance of this new photodetector are much higher than those of the others. The superimposed photodetctor is designed based on the standard 0.18 μm CMOS process. The responsivity, bandwidth and transient response of the photodetector are simulated by a commercial simulation software of ATLAS. The responsivities of two obtained photocurrent signals are 0.035 A/W and 0.034 A/W, while the bandwidths are 3.8 GHz and 5.2 GHz, respectively. A full differential optical receiver which uses the superimposed photodetector as input is simulated. The frequency response and 4 Gbit/s eye diagram of the optical receiver are also obtained. The results show that the two output signals can be used as the differential signal.