Simple rapid sensitive detection of tumor biomarker carcinoembryonic antigen (CEA) is of great importance for the screening, diagnosis and prognosis evaluation of various gastroenteric tumor. This paper presents a "turn-on" fluorescent biosensor based on molybdenum disulfide (MoS2) nanosheets and fluorophore labeled protein aptamer for rapid and sensitive detection of CEA protein. CEA aptamer probe can be adsorbed on the surface of MoS₂nanosheets in close proximity via van der Waals force, triggering fluorescence resonance energy transfer, and consequently fluorescence signal of aptamer probe was quenched. While in the presence of CEA protein, the fluorescence signal was recovered because aptamer probe could detach from MoS₂nanosheets with binding induce conformation change. MoS₂nanosheets with high quenching efficiency combined with well discrimination ability between aptamer and aptamer/protein afforded the biosensor easy construction, fast detection and high sensitivity. The sensing platform also exhibited good reproducibility, selectivity and showed high sensitivity for CEA protein in a broad range of 100 pg/mL-100 ng/mL with the detection limit of 34 pg/mL. The aptamer-MoS₂based fluorescent biosensor may be an ideal mode for protein detection in clinical sample, pesticide detection and environmental monitoring.