Microbial enhanced oil recovery has been well acknowledged and becoming an advanced technology for oil recovery. Compared with current techniques, a newly technical strategy of the in-situ heavy oil gasification to methane for oil viscosity reduction was proposed and successfully proved via enriching the methanogenic consortia from the brine of oil reservoir with heavy oil as carbon source. During 200 days anaerobic culturing, 2.34 g of heavy oil was degraded coupling with 1514 lmol of methane production. Phylogenetic diversity analysis showed that the enriched consortia composed with sequences affiliated with the Firmicutes, Proteobacteria, Deferribacteres and Bacteroidetes. The recovered archaeal phylotypes were close to the Methanobacteriales and Methanosarcinales, which could convert the produced small molecules (formic acid and acetic acids) to methane. The viscosity of the degraded heavy oil was reduced by 72.45% at 20 MPa after the dissolution of the produced methane. Core flooding tests finally showed that the oil degradation and methane production by the enriched methanogenic consortia made 14.7% of the tertiary enhanced oil recovery. These results demonstrated a promising and practical strategy of microbial technology on oil recovery by activating the methanogens in heavy oil reservoir.