There is an increasing interest in grapheme photodetector for its applications, because graphene has rich optical and electronic properties, including zero band gap, high mobility and special optical absorption properties. A design of microcavity-enhanced photodetector based on ultra-thin graphene is proposed in this paper: detector absorption can be effectively improved by confining the light field in the microcavity. Through studying the light field resonant condition in the microcavity and enhanced mechanism of detector responsivity under resonant mode, the light absorption model of a microcavity-enhanced graphene photodetector under standing wave effect is established; it is analyzed that the influences of microcavity mirror reflectivity and length on detector performance are increased by light field. Further the optimal structure parameters and performance evaluations of microcavity-enhanced graphene photodetector at different incident wavelengths are demonstrated. Theoretical analysis shows that under the standing wave effect the effective absorption coefficient of monolayer graphene at the antinode is one multiple enlargement compared with no cavity; the microcavity length and topbottom mirror reflectivity directly affect the optical total phase during light folding back at one time in the microcavity, and the shift of the total optical phase changes the full width at half maximum (FWHM) of the responsivity of the microcavity-enhanced graphene photodetector. Through coordinating the relations among the microcavity length and reflectivities of two mirrors and the incident wavelength, it can be realized that the photodetector has a good characteristic of wavelength selectivity. At a nominal operating wavelength of 850 nm, the presented microcavity-enhanced graphene photodetector can reach a responsivity of 0.5 A/W, 32-fold increase compared with monolayer graphene photodetector with no cavity and FWHM can reach 10 nm, indicating that the designed photodetector has a high responsivity and a good charactoristic of narrowband. As for the application in the practical engineering, through adopting bias on the two sides of graphene in the cavity to speed up the migration velocity of the photon-generated carrier, more photon-generated carriers are produced to increase the photodetector responsivity. However, the increased level of photodetector responsivity will be impeded acctually on account of the high contact resistance between graphene and electrode, and the measured value will not equal the theoretical value, so the quantitative analysis on the value of the bias should be carried out. Through combining the microcavity with graphene the incident light can be confined to reflect multiple times between two mirrors in the microcavity to improve the graphene absorption, and then make the microcavity-enhanced graphene photodetector responsivity improved. Our approach can be used to improve the optical response of graphene photodetector, and provides a way to solve the trade-off between photodetector responsivity and response speed.