NOVELTY - Preparing biological organic fertilizer using sludge from sewage factory, involves mixing e.g. ferment strain powder, and bran to perform fermentation, agitating and maintaining oxygen supply, again fermenting to obtain proliferated strain, preparing medium by mixing sludge and crop straw powder, adding proliferated strain to perform aerobic fermentation, fermenting sludge until white or grey white powder material forms, processing granulation procedure, crushing the fermented sludge, performing secondary granulation, sieving and packaging to obtain the product.    USE - The organic fertilizer is useful for improving soil granular structure, soil environment and quality of crops.    ADVANTAGE - The fertilizer does not cause secondary pollution.    DETAILED DESCRIPTION - Method for preparing biological organic fertilizer using sludge, involves (a) uniformly mixing (in kg): ferment strain powder (10), bran (1000), calcium-magnesium phosphate fertilizer (40) and brown sugar (20) with water (300), adding the mixture into fermentation tank to performing aerobic fermentation, (b) agitating for one time, each time 24 hours and maintaining oxygen supply, fermenting at 58-62 degrees C for 96 hours and then reducing the temperature, again fermenting for 120-144 hours and obtaining the proliferated strain from the fermentation tank, and drying the strain, (c) preparing fermentation culture medium by mixing sludge from city sewage processing factory and crop straw powder in the weight ratio of 1:1, and adding proliferated strain (0.3%) to process aerobic fermentation in the fermentation tank, (d) turning over for one time in each day, controlling temperature at 68-70 degrees C and fermenting for 7-8 days, reducing the temperature to 20-30 degrees C and reducing the relative humidity to 38-40%, fermenting sludge until white or grey white powder material is obtained which has no odor, and processing granulation procedure after fermentation, and (e) crushing the fermented sludge, performing secondary granulation, processing secondary parching, cooling the processed sludge, sieving by a sieve of 60 meshes, measuring and packaging to obtain the product, where the parching temperature is 80 degrees C and the time of each time of parching is 20-30 minutes, and the cooling temperature is 20-25 degrees C.