The invention relates to a marine oligosaccharide biological agent improving crop saltresistance and a preparing method thereof. The marine oligosaccharide biological agent improving the crop saltresistance is prepared from chito-oligosaccharide of which the molecular weight is 3000-4000, asparagus agaro-oligo saccharide, bacillus mucilaginosus, calcium chloride and water, wherein the weight ratio of chito-oligosaccharide and asparagus agaro-oligo saccharide is (1-3):0.5, or preferably 1:0.5; calcium chloride is 10-20 mmol/L, the content of oligosaccharide is larger than or equal to 0.2%, and the living bacteria count is larger than or equal to 2 hundred million/mL. The preparing method comprises the steps of 1, preparing a chito-oligosaccharide solution; 2, preparing bacillus mucilaginosus fermentation liquor; 3, adding the prepared chito-oligosaccharide solution, asparagus agaro-oligo saccharide and calcium chloride to the bacillus mucilaginosus fermentation liquor according to a certain proportion at a room temperature, and performing stirring and mixing evenly to obtain the marine oligosaccharide biological agent improving the crop saltresistance. The marine oligosaccharide biological agent improving the crop saltresistance is used for promoting pepper seed germination under salt stress, can obviously increase the germination rate of pepper seeds, promote the growth and development at the seedling stage, enhance the resistance capacity of pepper seedlings to the salt stress, and can also be applied to promotion of the seed germination and seedling growth of other crops of cucumbers and paddy rice and the like.