The invention provides a preparation method of overbased calcium sulfurized alkyl phenate. The preparation method comprises the following steps: (A) carrying out catalytic hydrogenation on a nutshell extract product and distilling to obtain m-Pentadecylphenol; (B) carrying out alkylation between 0.01-100 wt% of m-Pentadecylphenol and alpha-olefins to obtain dialkylphenol; (C) mixing the substances obtained in the above two steps with base oil, a promoter, a cosurfactant, a composite cure activator, lime and sulphur, and carrying out a neutralizing sulfuration reaction to obtain a first product; (D) adding the first product into a promoter and lime and carrying out a carbonatation reaction by introducing carbon dioxide to obtain a second product; and (E) carrying out reduced pressure distillation on the second product and filtering to obtain the product overbased calcium sulfurized alkyl phenate. The preparation method has advantages as follows: without using high-boiling point alcohol and solvent naphtha as a solvent, post-treatment cost is reduced; and the product obtained has high base number and small viscosity.