Due to the weak absorption and low light-matter interaction of MoS₂, intrinsic MoS₂ photodetector usually has low photoresponse, thus limiting its real application. Herein, MoS₂ homojunction was constructed by using the chemical vapor deposition grown intrinsic MoS₂ films and the Nb-doped MoS₂ films. The results show that the Nb doping will induce p-type doping in MoS₂, where the electron concentration will decrease by 2.08 × 10¹² cm^–2 after Nb doping. By investigating the photoelectric effect of MoS₂/Nb-doped MoS₂ homojunction-based phototransistor, the tunability of the photoresponse, detectivity as the function of the external field, wavelength, and power of light have been studied in detail. The results show that the photoresponse and detectivity are strongly dependent on the gate voltage due to the external field tuned interlayer photoexcitation attributing to the band bending. The maximum of photoresponse can reach 51.4 A/W, the detectivity can reach 3.0 × 10¹² Jones, which is two orders higher than that of intrinsic MoS₂. Furthermore, by correlating the photoresponse and detectivity with the external field, it is found that the photodetection of MoS₂ homojunction can be significantly tuned and exhibit well photodetection in infrared. This comprehensive work not only sheds light on the tunable photoexcitation mechanism but also offers a strategy to achieve a high-performance photodetector.