NOVELTY - Preparing biomass oil by (a) performing solid-liquid separation of waste lees liquid and filtering, and mixing filtrate with wastewater or clean water, (b) inoculating mixture with single or mixed algae strains, subjecting mixture to solar light, and harvesting algae, (c) performing catalysis of harvested algae, (d) adding solvent e.g. hexane and toluene, extracting mixture and separating oil-water mixture and obtaining biomass oil as oil phase, (e) mixing aqueous phase obtained in step (d) and waste lees liquid, repeating steps (a)-(d), and continuously preparing biomass oil.    USE - The method is useful for preparing biomass oil (claimed).    ADVANTAGE - The method reduces energy consumption, is cost-effective, and utilizes waste water and lees.    DETAILED DESCRIPTION - Preparation method of biomass oil involves (a) performing solid-liquid separation of waste lees liquid by centrifuging or extruding and filtering, mixing filtrate with wastewater or clean water, and regulating chemical oxygen demand of mixture to 50-15000 mg/l, total organic carbon concentration to 50-5000 mg/l, ammonia concentration to 3-800 mg/l, total nitrogen concentration to 10-1200 mg/l, total phosphorus concentration to l-200 mg/l, and pH to 6-9 for making it suitable for growing all types of algae, (b) adding mixture into open or closed reactor and inoculating with single or mixed algae strains, maintaining temperature at 15-45 degrees C, illumination intensity at 3000-12000 lux, laminar flow rate of 3-60 cm/s in the reactor, and concentration of carbon dioxide containing gas at 3-25% to ensure enough carbon source for growing algae, subjecting mixture to solar light after algae growth reaches peak, and harvesting algae using belt drier method or centrifugation when solid algae concentration reaches 5-50%, (c) placing mixture in a stainless steel reactor, adding 0.5-5% catalyst and performing reaction at 180-350 degrees C and 0.5-20 MPa for 5-120 minutes, (d) allowing mixture to sediment, adding solvent chosen from hexane, dichloromethane, toluene, dimethylbenzene, methanol, propanol and butanol to supernatant, extracting mixture and separating oil-water mixture and obtaining biomass oil as oil phase, (e) mixing aqueous phase obtained in step (d) and waste lees liquid, repeating steps (a)-(d), and continuously preparing biomass oil.