The invention relates to a diesel oil additive. The diesel oil additive comprises, by weight, 20-80 parts of a cetane number improver, 3-40 parts of a tension ring caged hydrocarbon pentacyclic[5.4.0.0<2.6>.0<3.10>.0<5.9>]hendecane and 5-50 parts of assistant, and the cetane number improver is alkyl nitrate. The assistant can be one or a mixture comprising more of a solvent, a surfactant, a pour-point depressant, a cleaning dispersant, a smoke eliminating and combustion supporting agent and a stabilizer. The diesel oil additive has the advantages of simple production technology, low price, realization of saving of oil by 1-5% after the addition of 0.1-0.6wt% of the additive to diesel oil, reduction of the oil consumption of diesel engines, effectively reduction of the emission of harmful substances in tail gas, and good use effect.