MoS ₂ -graphene-based hybrid structures are biocompatible and useful in the field of biosensors. Herein, we propose a heterostructured MoS ₂ /aluminum (Al) film/MoS ₂ /graphene as a highly sensitive surface plasmon resonance (SPR) biosensor based on the Otto configuration. The sensitivity of the proposed biosensor is enhanced by using three methods. First, prisms of different refractive index have been discussed and it is found that sensitivity can be enhanced by using a low refractive index prism. Second, the influence of the thickness of the air layer on the sensitivity is analyzed and the optimal thickness of air is obtained. Finally, the sensitivity improvement and mechanism by using molybdenum disulfide (MoS ₂ )-graphene hybrid structure is revealed. The maximum sensitivity ~190.83°/RIU is obtained with six layers of MoS ₂ coating on both surfaces of Al thin film.