The bottom ash residue derived from biomass power generation was characterized by alkalinity and contained about 35 mineral elements, which offered a possibility to improve anaerobic digestion of lignocellulose biomass via providing mineral nutrients for microorganisms and alkali conditions for pretreatment. Technically, bottom ash was mixed with distilled water to prepare the extracting solution for this investigation. To check the function as a nutrient provider of bottom ash, the increased bottom ash loading from 0.37 to 2.96% was employed to achieve the extracting solutions with different concentrations of mineral elements. Rice straw was mixed with these extracting solutions for 7 days and then employed for the anaerobic digestion in batch. Results indicated that 314.38 mL/g of total solid biogas was achieved by 0.37% bottom ash, which was 21.2 and 15.8% higher than that of the blank and 0.12% NaOH (equivalent alkalinity of 0.37% bottom ash), respectively, proving that bottom ash substantially functioned as a nutrient provider with the resultant improvement. However, the decreased biogas production at higher loadings suggested potential inhibitions. Besides, to check the potential function of alkali pretreatment using bottom ash, the soaking duration of rice straw in the extracting solution (the calculated bottom ash loading of 0.37%) was prolonged from 0 to 7 days to achieve the different intensities, and the results indicated that the lignin and hemicellulose removal was promoted to 20.1 and 25.3% after 7 days of pretreatment, respectively, and the corresponding biogas production and biomethane yield were increased by 63.1 and 28.3%, respectively, proving that the bottom ash can work as a base provider for alkali-pretreating rice straw to facilitate anaerobic digestion.