Microwave absorbing materials are widely used in military and civil fields. The current research focusses on novel microwave absorbing materials with intense absorption, wide bandwidth, light weight and thin thickness. Black phosphorus (BP) and sulfur-doped black phosphorus (BP-S) crystals were prepared at a high temperature and a high pressure, and then BP and BP-S nanosheets were obtained via liquid-phase exfoliation. Compared with BP, the complex permittivity and εr″/εr′ of BP-S are enhanced at 1.0-16.5 GHz. The minimum reflection loss (RL) reaches -41.4 dB at 9.6 GHz, and the RL exceeding -10 dB in a frequency range of 7.0 to 16.1 GHz is obtained for the absorber thickness of 1.0-2.0 mm. The minimum RL frequency dependence of the absorber thickness for BP-S comply with a λ/4 matching model. BP-S has broad application prospects as a high-performance microwave absorbing material.