In recent years, with the rapid development of transfer technologies related to graphene and other two-dimensional layered materials (2DLMs), graphene sandwiched 2DLMs have been confirmed to be outstanding tunneling and optoelectronic devices. Here, compared to the planar SnSe2-Au device, the SnSe2 device with different thicknesses (12-256 nm) is incorporated into graphene sandwiched structures for photodetection. The results indicate that the photoresponse properties are dependent on the thickness and gate voltage. In particular, under 532 nm illumination and at a Vg of +80 V, the SnSe2 device with a thickness of 96.5 nm shows an impressively high responsivity of 1.3 × 103 A W-1, an external quantum efficiency of 3 × 105%, and a detectivity of 1.2 × 1012 Jones. Besides, a high response speed (a rise time of 30.2 ms and a decay time of 27.2 ms) and flat photoswitching behavior are achieved without the gate voltage. In addition, the intrinsic mechanisms are further discussed through the relative spatial potential difference and the band alignment diagrams of the graphene-SnSe2-graphene and Au-SnSe2-Au structures. These findings indicate that SnSe2 has great potential for practical applications in next generation high performance optoelectronics.