Photoacoustic (PA) imaging, as a fast growing technology that combines the high contrast of light and large penetration depth of ultrasound, has demonstrated great potential for molecular imaging of cancer. However, PA molecular imaging of orthotopic brain tumors is still challenging, partially due to the limited options and insufficient sensitivity of available PA molecular probes. Here, the direct formation of single-layer (S-MoS2), few-layer (F-MoS2), and multi-layer (M-MoS2) nanosheets by the albumin-assisted exfoliation without further surface modifications is reported. It is demonstrated that the PA effect of the MoS2 nanosheets is highly dependent on their layered nanostructures. Decreasing the number of nanosheet layers from M-MoS2 to S-MoS2 can both significantly enhance the near-infrared light absorption and improve the elastic properties of the nanomaterial, resulting in greatly amplified PA effect. The in vitro experiments demonstrate that the prepared S-MoS2 with excellent biocompatibility can be efficiently internalized into U87 glioma cells, producing strong PA signals for highly sensitive detection of brain tumor cells, with a detection limit of approximate to 100 cells. Intravenous administration of S-MoS2 to both U87 subcutaneous and orthotopic tumor-bearing mice shows highly efficient tumor retention and significantly enhanced PA contrast. Tumor tissue approximate to 1.5 mm below the skull can still be clearly visualized in vivo. Previous studies suggest that the fabricated S-MoS2 with amplified PA effect have high potential to serve as an efficient nanoplatform for sensitive PA molecular imaging and hold promising prospect for translational medicine.