A simple perfect absorption structure is proposed to achieve the high efficiency light absorption of monolayer molybdenum disulfide (MoS2) by the critical coupling mechanism of guided resonances. The results of numerical simulation and theoretical analysis show that the light absorption in this atomically thin layer can be as high as 98.3% at the visible wavelengths, which is over 12 times more than that of a bare monolayer MoS2. In addition, the operating wavelength can be tuned flexibly by adjusting the radius of the air hole and the thickness of the dielectric layers, which is of great practical significance to improve the efficiency and selectivity of the absorption in monolayer MoS2. The novel idea of using critical coupling to enhance the light-MoS2 interaction can be also adopted in other atomically thin materials. The meaningful improvement and tunability of the absorption in monolayer MoS2 provides a good prospect for the realization of high-performance MoS2-based optoelectronic applications, such as photodetection and photoluminescence.