In this paper, we design a plasmonic perfect absorber based on black phosphorus (BP) with enhanced terahertz modulation. By tuning the chemical potential (μc) of BP, the modulation depth can reach up to 95%. The influence of geometric size and bandgap of BP on reflection spectra is also investigated. Moreover, the effect of the incident angle on the reflectance is discussed with different values of μc. Our results show that the plasmonic nanoslit mode contributes to the enhancement of the modulation effect. This simple periodical structure provides a potential route to design a tunable plasmonic BP-based modulator in the THz range.