The invention discloses an elastic agent for well cementation, a preparation method thereof and an elastic isolation fluid, and relates to the field of petroleum drilling engineering. The elastic agent is prepared from raw materials comprising the following components, in parts by weight: 20-35 parts of bisphenol A, 10-25 parts of epichlorohydrin, 10-12 parts of diglycidyl ether, 0.1-0.3 part of sodium hydroxide, 10-20 parts of diethylenetriamine, 20-30 parts of propylene carbonate, 15-30 parts of amino polyether, 10-20 parts of dimethylimidazole, and 5-15 parts of a foaming agent. The elastic agent is not smashed under a high temperature of 150 DEG C and a pressure of 70MPa, thus when the elastic isolation fluid is introduced to the bottom of a high-temperature and high-pressure well for displacement, final effects of the elastic isolation fluid is guaranteed to not be influenced by in-advance gas releasing caused by particle smashing. The elastic isolation fluid containing the elastic agent can be used within a temperature raising range of 0 DEG C to 150 DEG C under closed environments, the applicable temperature difference range between a mudline temperature and a bottom hole temperature is wider, the elastic isolation fluid can be used for all stages of sleeve annular space, and guarantees annular space pressure increasment to be less than largest anti-collapse strength of each stage of the sleeves.