Abstract: Most of the oil and gas hydrate reservoirs discovered today are sandstone formations, and porosity as an important parameter of rock core is well worth studying. There are many methods to study rock porosity at present, but most of them have obvious defects. However, the low-field nuclear magnetic resonance(NMR) is applied to samples to explore the pore structure of rock without any damage to the sample. At the same time, low-field NMR can also be applied to undercompacted samples. The rock can be measured by LF NMR to obtain the corresponding pore size distribution and the porosity of the entire sample and the classification pore. Stratum structure will change with long-term temperature changes. Therefore, this experiment simulated the change of porosity of the whole rock and the classified pores under different temperature conditions when the sandstone freezes and thaws in the strata. The experimental results show that the overall porosity of rock is closely related to the porosity of macropores and mesopores. In a freeze-thaw process, there is an overcooling phenomenon in the pore water phase transition process, indicating that the supercooling phenomenon has a great influence on the porosity change of the rock.