The method provides a method for preparing biogas from sludge and straw materials by mixed fermentation. The method comprises the following steps: mixing sludge and straw materials, controlling the C/N value of the mixed material of sludge and straw material within 10 to 25, adding the mixed material into an anaerobic digester, adding anaerobic active sludge at an amount of 10 to 100% the total weight of the mixed material, inoculating, diluting with water to obtain the TS content of the diluted material within 5 to 10%, anaerobically fermenting for 15 to 20 days, and collecting the biogas obtained by fermentation. The method solves the problem that the material in the fermentation system is not suitable for anaerobic digestion due to the lower C/N ratio, thus improving the actual decomposition rate and the gas output of sludge and achieving considerable application and promotion values.