NOVELTY - Agar original algal biological fertilizer comprises red algae having a cell wall polysaccharide compound (I) as raw material, where the liquid biological fertilizer containing oligosaccharide elicitor is prepared by combined treatment of superfine grinding and bacteria degradation technology.    USE - The biological fertilizer is useful for promoting growth and stress resistance of plant.    ADVANTAGE - The biological fertilizer: has natural green safe product; is absorbed easily by the plant and devoid of any chemical substances; can improve the plant germination rate; accelerates plant growth; and increases plant resilience.    DETAILED DESCRIPTION - Biological fertilizer, comprises red algae having a cell wall polysaccharide compound of formula (I) as raw material, where the liquid biological fertilizer containing oligosaccharide elicitor is prepared by combined treatment of superfine grinding and bacteria degradation technology.    n = positive integer.    An INDEPENDENT CLAIM is included for preparing biological fertilizer comprising (a) disposing the agar original alga by a micro-grinding, and getting the superfine algae powder of the particle size to 0.2-1 mu m, (b) adding the superfine algae powder into water to the volume ratio of 1:2-1:3, and digesting the powder at 50-60 degrees C for 12-24 hours, (c) activating the agar degradation bacteria, shaking cultivation at 25-28 degrees C for 24-36 hours, and making the bacteria density in the obtained fermentation bacteria liquid reach of 18-32 g/l (wet weight), where the activity of the agar degradation bacteria in the fermentation bacteria liquid reaches 250-350 U/ml, (d) putting the digested algal liquid and frond into the cultivated fermentation bacteria liquid for bacteria solution, where the volume ratio of the algal liquid to the fermentation bacteria liquid is 1:10-1:20, and cultivating the solution at 25-36 degrees C for 24-36 hours and then separating the supernatant fluid, (e) adding the supernatant fluid to the volume ratio of 1:1-2 into the new fermentation bacteria liquid after cultivating for 24-36 hours, and performing the secondary bacteria solution, where the time is 36-48 hours, and (f) finally sterilizing the bacteria solution liquid.