NOVELTY - Preparing methane comprises collecting fresh Enteromorpha and then drying, soaking the Enteromorpha into water and then pulping, adding water into slurry to obtain Enteromorpha biomass degradation liquid, collecting and fermenting straw solid compost to obtain a straw biomass degradation matrix, compounding the Enteromorpha biomass degradation liquid and the Enteromorpha degradation matrix, placing the acidized matrix into a methane fermenting device, conducting water sealing, and fermenting the methane with constant pressure.    USE - The method is useful for preparing methane (claimed).    ADVANTAGE - The method ensures that the enteromorpha/ and straws are complemented with each other, so that the buffering capacity of the process is improved, the social and economical benefits are good, and the two-step fermentation process is implemented with quick fermentation starting, reduced methane fermentation period and high methane generating amount. The method prepares methane with increased utilization ratio and reduced energy consumption, and realizes the resource utilization of the Enteromorpha/ and the straws, improves the sea environment, reduces the environment event formed by burning the straws, produces the clean methane energy resource and achieves reducing emission.    DETAILED DESCRIPTION - Preparing methane comprises collecting fresh Enteromorpha and then drying, soaking the Enteromorpha into water and then pulping, adding water (20-25%) into slurry, fermenting the Enteromorpha at 30\|2?oC, 8 volume versus mass (vvm) for 48-72 hours to obtain Enteromorpha biomass degradation liquid, liquid fermenting the composite degradation bacteria I, collecting and fermenting straw solid compost to obtain a straw biomass degradation matrix and then crushing into small section (0.2-0.5 cm), adding water to soak over night, adjusting the water content to 45-55 wt.%, liquid fermenting the composite degradation bacteria II for 2-4 days, compounding the Enteromorpha biomass degradation liquid and the Enteromorpha degradation matrix with mass ratio of 3: 2, adding fresh water or biogas slurry to obtained mixture and then hardening and tempering, adding acidified sludge (30 wt.%) to the mixture and then adding fresh water or biogas slurry to the solid mass contents (8-12 wt.%) at 35\|1?oC, stirring for two times for 30 minutes and then acidifying for 2 days to obtain matrix, adding sodium alginate and sea-tangle and then centrifuging using a filter, adding wastewater into an aeration tank and then collecting sediment (40%), adding fresh water or a biogas fermentation substrate to the solids content (8-12 wt.%) to obtain acidized sludge, placing the acidized matrix into a methane fermenting device using marsh gas (30 wt.), adding sodium carbonate or plant ash adjusting fermentation substrate with pH of 7.2-7.5 at 35\|1?oC to perform anaerobic fermentation of marsh gas, conducting water sealing, and fermenting the methane with constant pressure, supplementing materials and discharging slag to perform fermentation when the methane generating amount is smaller than average methane (5%).