Abstract: Four kinds of different calcium alkyl salicylate additives were mixed with base oil ，respectively，to test their water separation performance according to the method of SH/T 0619. The results find an obvious difference in water separation property of the four additives. The phenomena were discussed from average molecular weight and the alkyl chain structure. The analysis indicates that the alkyl carbon chain branching degree of calcium alkyl salicylates is the main reason for the difference. The less the branching degree is, the better the water separation performance is. There is little relationship between the average molecular weight and the water separation property.