The invention relates to a preparation method of light-color pentadiene petroleum resins. The method is characterized in that isoprene and dicyclopentadiene are separated from C5 which is the byproduct of ethylene pyrolysis, the obtained distillate the main component of which is pentadiene is taken as a raw material, the content of the pentadiene is 60-80wt%, and chain-shaped or annular saturated hydrocarbon is taken as a reaction agent. The method comprises the following process flows: a, thermal polymerization: carrying out thermal polymerization reaction for 4-10 hours at 220-280 DEG C under the pressure of 2-8 MPa by taking pentadiene as the raw material; b, catalytic polymerization: using an immobilized Lewis acid catalyst to carry out catalytic polymerization for 2-8 hours at 600-200 DEG C under the pressure of 0.1-0.8 MPa by taking materials subject to thermal polymerization as raw materials and toluene or xylene as a solvent; c, catalyst removal: removing the catalyst by deposition and filtering; d, separation: carrying out steam stripping to obtain the non-reacted reactant, solvent and oligomeric C5 resin under vacuum at 220-300 DEG C, wherein the residual is the pentadiene petroleum resin; and e, granulation.