Waste reuse energy generation is encouraged in many countries. A low H/C ratio in coal is a limiting factor that affects biomethanation. However, corn straw has a better H/C ratio, which is beneficial to methane production. A series of biological gas production experiments was carried out using lignite B, bituminous coal D, bituminous coal C, and corn straw to study the effect of synergistic biodegradation of coal combined with corn straw into biomethane. Methane production was used as an index to analyze the gas production characteristics of single coal, single corn straw, and coal and corn straw. The results show that the optimum proportion of the conversion of lignite B and corn straw to biomethane is 2:1, and the optimum proportion of the conversion of bituminous coal D, bituminous coal C, and corn straw into biomethane is 3:1, and corn straw has a significant enhancement effect on the biological gas production in this experiment. When lignite B, bituminous coal D, and bituminous coal C are, respectively, mixed with corn straw, the highest methane production is 2.05, 2.69, and 1.35 mmol/g, indicating that corn straw has different promotion effects on gas production for coals of different rank: bituminous coal D + corn straw > bituminous coal C + corn straw > lignite B + corn straw. The stage change of COD and pH was consistent with that of the total gas production under optimal proportion, and the complementary advantages or mutual inhibition of methanogens was one reason for the differences in gas production between coal combined with corn straw and single coal.