In order to deeply study the imbibition mechanism of conglomerate reservoir, the law that fluid flows in different sizes of pores in the process of imbibition was researched by nuclear magnetic resonance. First, the conglomerate core was saturated by the simulated formation water, and then was saturated by polyfluoroethylene oil to form the irreducible water. During the experiment of imbibition, several time points were selected and the conglomerate core was tested by NMR. According to the relaxation time T2, pores were divided into large, middle and small three size ranges. As the imbibition time increases, the available rate of middle and small pores increase faster than large pores. The available rate of middle and small pores both exceed 50%, but the one of large pores is lower than 35%. The difference of mechanism between imbibition and waterflooding is explained by their NMR test results.