TECHNOLOGY FOCUS - BIOLOGY - Preferred Method: The method further comprises (iv) preparing biogas by anaerobic fermentation. In the step (iv), after hydrogen production, anaerobic fermenting and degrading algal cell to produce marsh gas. The production of biogas comprises (i) culturing Anabaena 7120 at constant temperature and continuous illumination, liquid ventilation culturing, storing the algae in solid agar BGII (+N) culture medium flat plate, (ii) making the algal cells to grow to a certain concentration in the culture medium, packing the sample into a glass tube, then maintaining the anaerobic environment, and sealing the test tube, and (iii) collecting algae cell after the photosynthetic hydrogen production process, then the algal sludge and seed sludge, adding distilled water and phosphate buffered saline buffer solution, sealing and placing in a water bath of 37 degrees C for fermenting and degrading to obtain biogas. The production of biogas specifically comprises (i) culturing Anabaena 7120 by taking BGII (+N) as liquid culture medium and subjecting to liquid culture, placing mixture at constant temperature of 30 degrees C in 30 mu mol photon m-2s-1, carrying out continuous illumination for 24 hours, passing through carbon dioxide-air mixed gas at a ratio of 2:98 (vol:vol), storing Anabaena 7120 after continuous subculture in agar concentration of 1.5% of solid BGII (+N) culture medium flat plate, where the storing condition is constant temperature of 30 degrees C in 30 mu mol photon m-2s-1 illumination intensity and air environment, (ii) dispensing algal solution into an anaerobic induction test tube, adding the photosynthetic oxygen inhibitor, introducing high pure argon gas continuously into the algal solution, under the anaerobic environment, sealing the test tube, placing the sample tube at 30 degrees C in 30 mu mol photon m-2s-1 light intensity and inducing algal cell for photosynthetic hydrogen-producing process, and (iii) collecting algal cell after photosynthetic hydrogen-producing process, taking 15 g fresh algal sludge, and 10 g seed sludge, adding distilled water and phosphate buffered saline buffer solution, sealing and placing in 37 degrees C water bath for fermenting and degrading to obtain biogas. Preferred Components: Blue algae is Anabaena 7120, of Institute of Hydrobiology, Chinese Academy of Sciences FACHB-418.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The photosynthetic hydrogen production inducer is dichlorophenyl dimethylurea. The inducer adding amount is 1 mu m.