High-concentration reduced graphene oxide (RGO) solution was produced using gelatin (Gel) as the stabilizer and subreductant and hydrazine hydrate (HHA) as the main reductant. The Gel-HHA-RGO nanosheets exhibited excellent colloidal dispersibility and stability in alkaline condition. The Gel-HHA-RGO filled natural rubber (NR) composites were prepared by water-based solution casting. Well-organized interconnected RGO networks were constructed throughout the NR matrix, which played an important role in determining the properties of composites. The tensile modulus and dynamic storage modulus were improved by several orders of magnitude with increasing RGO content. Meanwhile, a dramatic increase in electrical conductivity with a low percolation threshold of 0.21 vol % was perceived. Strain-sensing tests revealed that the RGO/NR composites exhibited outstanding strain sensitivity and repeatability, which could be used to detect the cyclic movements of human joints. The results are promising, in the rubber industry to guide the fabrication of highly sensitive and stretchable strain sensors for engineering application.