NOVELTY - Culturing self-autotrophy freshwater microalgae and purifying marsh gas by biogas liquid, involves methane fermentation: fermenting organic substance to obtain marsh gas/biogas slurry; biogas liquid processing: filtering the liquid to remove residual solid particle, diluting the liquid by water till concentration of biogas liquid to 10-100% as the autotrophy freshwater microalgae full nutrient culture medium; vaccinating autotrophy freshwater microalgae and introducing the marsh gas: filling the processed liquid into photobioreactor for culturing self-autotrophy freshwater microalgae.    USE - For culturing self-autotrophy freshwater microalgae and purifying marsh gas (claimed).    ADVANTAGE - The method uses the biogas fermenting culture base microalgae from autotrophy fresh water, at the same time in the marsh gas is carbon dioxide as the autotrophy freshwater microalgae, it solves the autotrophy freshwater microalgae culture cost, but also improves the purity and quality of marsh gas, and reduces the environment pollution, biogas slurry to improve the environment and increase the economic, social and ecological benefits.    DETAILED DESCRIPTION - Culturing self-autotrophy freshwater microalgae and purifying marsh gas by biogas liquid, involves methane fermentation: fermenting the organic substance according to the existing method to obtain marsh gas and biogas slurry; biogas liquid processing: filtering the obtained liquid to remove the residual solid particle, diluting the liquid by water till the concentration of the biogas liquid to 10-100% as the autotrophy freshwater microalgae full nutrient culture medium; vaccinating the autotrophy freshwater microalgae and introducing the marsh gas: filling the processed liquid into the photobioreactor for culturing the self-autotrophy freshwater microalgae, accessing the fresh self-autotrophy freshwater microalgae and leading the methane into the photobioreactor from the bottom of the reactor; microalgae fixing carbon dioxide: using the carbon dioxide in the methane as carbon source, conducting photosynthesis self-length carbon dioxide for culturing microalgae mode, controlling the culturing temperature of microalgae at 20-30 degrees C, illumination intensity as follows: at the early culturing time of the 1-3 days, illumination intensity of 1000-2000Lx, increasing the light intensity to 4000-8000Lx at the second stage for the 4-12 days, 12 hours/day illumination; obtaining the self-autotrophy fresh harvesting micro-algae as the raw material of the biogas fermentation: after culturing for 2-4 weeks, themicro-algae stopping growing, stopping illumination and leading gas, taking air floating to the algae solution, the upper thicker microalgae concentrated as biogas fermentation raw material and the lower diluted algae solution is as the seed algae liquid.