The invention discloses a microbial inoculum for bioremediating a coastline polluted by oil spilling and a preparation method thereof. The microbial inoculum for bioremediating the coastline polluted by oil spilling is made of mixed fermentation liquor and a fixed carrier as raw materials, wherein the mixed fermentation liquor comprises bacillus pumilus CGMCCnO.3384 fermentation liquor, Venice acinetobacter CGMCCnO.3385 fermentation liquor and/or Doxa pseudomonad CGMCCnO.3386 fermentation liquor and/or Ralstonia pickettii CGMCCnO.3387 fermentation liquor. By using the synergic action of different strains on the degradation of various petroleum hydrocarbons, the microbial inoculum improves the degradation speed of petroleum hydrocarbon pollutants and thoroughly degrades various petroleum hydrocarbon pollutants; in addition, a flora formed by a plurality of strains of bacteria has more stable structure and can better adapt to various petroleum-polluted environments. The microbial inoculum rapidly grows on the polluted areas, forms competitive advantages and remarkably increases the quantity of hydrocarbon degraded microbes in the polluted areas, thereby improving the biodegradation rate of crude oil.