Abstract: In order to deeply analyze the complex physical property changes exhibited by multi-layer sandstone oil fields after long-term water injection development, this paper adopts the sand filled tube nuclear magnetic online water flooding method, and conducts nuclear magnetic detection while high magnification water flooding to observe the micro-pore structure and physical property changes of long-term water flooding reservoirs. Research has shown that due to the significant differences in the content of fine particles such as siltstone and mud in different reservoirs, there are different patterns of porosity and permeability changes during water injection development. Among them, Class I reservoirs have low content of siltstone and mud, and the migration of particles after water injection flushing gradually increases the porosity and permeability of the reservoir. Class III and IV reservoirs have high content of siltstone and mud, and the expansion of clay after water injection leads to a gradual decrease in porosity and permeability. Class II reservoirs are between the two reservoirs above, with little change in porosity and permeability, which can guide the reasonable prediction of water drive development effects in different reservoirs of oil fields.