The invention relates to an oil displacement agent for improving recovery ratio technology in the process of oilfield development, in particular to a bioactivity oil displacement agent for improving recovery ratio in ultra-low permeability oilfields; the bioactivity oil displacement agent is characterized in that: the bioactivity oil displacement agent is prepared by the following ingredients in percentage by weight: 0.3-0.5 percent of SUN biological enzyme, 0.02-0.04 percent of SUN-B4, 0.01-0.02 percent of formaldehyde, 0.3-0.5 percent of APG1214 and the balance of water, wherein the APG1214is alkyl polyglucoside, the SUN-B4 is formed by mixing scale-prevention antisludging agent, enzyme activator and water according to weight ratio of 15:5:80; the oil displacement agent has hypersalinity resistance and high calcium ion resistance, is non-corrosive to equipment, is harmless to environment, has high displacement efficiency in the high-hypersalinity (10*104mg/l) high-calcium (2000mg/1) complex water system, and has low and stable oil-water interface stress, and the displacement efficiency can be improved by more than 15 percent when the concentration of the injected oil displacement agent reaches 700mg/1h.