Understanding the mechanisms underlying photocurrent generation in graphene-based photodetectors is essential for enhancing their spectral selectivity and response stability. Therefore, this study aims to systematically investigate and compare the photoresponse mechanisms of pristine monolayer graphene and CsPbBr3 quantum dot (QD)-sensitized graphene heterostructures under 450 nm and 525 nm laser irradiation. Pristine graphene exhibits a wavelength-dependent photoresponse, with a pronounced negative photocurrent (NPC) under 450 nm irradiation due to photodesorption of p-type dopants and defect-assisted photoionization from the SiO2 substrate. Under 525 nm irradiation, where photon energy is insufficient to activate surface and substrate mechanisms, graphene exhibited an unstable positive photocurrent (PPC) driven by weak photoconduction. In contrast, the CsPbBr3/graphene heterostructure consistently exhibits strong and stable PPC at wavelengths. The integration of CsPbBr3 QD enables efficient photocarrier generation and transfer to the graphene channel, overriding parasitic NPC pathways and stabilizing the overall photoresponse. Time-resolved measurements reveals significantly faster rise and fall times in the heterostructure, confirming a transition from slow surface-mediated processes to rapid photoconductive dynamics. This comparative study elucidates how intrinsic graphene properties, substrate interactions, and heterojunction effects collectively drive the photocurrent polarity and enhance performance in hybrid 2D/0D photodetectors.