The invention discloses a bio-based high-cetane-number diesel additive, a preparing method thereof and a clean diesel composition. The clean diesel additive is prepared from levulinic acid ester, polymethoxy dimethyl ether, dilinoleic acid, tert-butylferrocene and 2,6 butylated hydroxytoluene. Compared with the prior art, through the clean diesel additive, discharging of particulate pollutants andblack carbon smoke can be obviously reduced, a cetane-number accelerant such as isoamyl nitrate does not need to be added, deployed diesel is stable, and does not contain a nitro compound, discharging of tail gas NOx is not additionally increased, and an environmentally-friendly effect is achieved; the bio-based high-cetane-number diesel additive has the advantages of being high in biomass-carbonusing degree, good in lubricity and high in flash point, particularly, the additive has a good coagulation reduction effect on low-grade diesel; meanwhile, the cetane number of the diesel can be improved, a knocking effect is reduced, and the bio-based high-cetane-number diesel additive can be used in high-standard diesel machines.