As fuel additives, lanthanum and cerium are the most active rare earth elements. In order to compare the emission reduction performance of the two rare earth elements additives, lanthanum oxalate and cerium oxalate are used as the comparison objects in this paper. In order to reduce the error, it is necessary to prepare lanthanum oxalate and cerium oxalate. After lanthanum oxalate and cerium oxalate was prepared by precipitation method, a series of modern characterization methods, such as XRD, SEM, EDS and BET, are used to characterize and analyze the self-made additive powder, and the relationship between the emission reduction performance and the internal relationship is analyzed. By changing the ratio of fuel additives and carrying out the emission test on the self-built diesel engine platform, the optimal amount of additives is obtained. Fuel additives of lanthanum oxalate and cerium oxalate with a mass fraction of about 45mg/L for diesel engine is a more suitable choice. Cerium oxalate has better emission reduction performance than lanthanum oxalate. The conclusion is that reasonable use of two organic acid rare earth fuel additives can greatly reduce NO_X, CO and HC emissions from diesel engines.