The tight sandstone reservoirs are characterized by small pore throats，strong hydrophilicity，and well-developed micro-fractures，etc. The capillary imbibition phenomenon is prone to occur in the reservoir under the action of a large capillary force. In order to reveal the microscopic distribution of capillary imbibition of water in the pore network，the tight sandstones of Penglaizhen Formation in western Sichuan are selected to conduct the vertical spontaneous imbibition experiments under simulated formation water conditions，and the distribution and variation laws of the fluid during the imbibition process are studied by using the nuclear magnetic resonance（NMR）technology. The results show that when the spontaneous imbibition lasts for 5 minutes，the self-imbibition water is mainly concentrated in the nano-scale pores of 0-0.1 μm，accounting for more than 84%. With the increase of spontaneous imbibition time，the proportion of self-imbibition water distributed in the nano-scale pores of 0-0.1 μm gradually decreases，and the proportion of self-imbibition water distributed in the pores of submicron level of 0.1-1 μm and micron level of 1-10 μm gradually increases，in which the maximum increase of self-imbibition water in submicron pores of 0.1-1 μm increases from 11% to 25%. The number of micron pores with a radius greater than 10 μm is small，and the capillary force is extremely weak，resulting in a low degree of self-imbibition water. The largest proportion is only 1.95%. There is no obvious upward or downward trend in the proportion of self-imbibition water. When the external positive pressure difference is not considered，the capillary imbibition phenomenon will preferentially occur in nano-scale pores. Pore radius size，pore type，proportion of different pore radius，water saturation，etc.are the main factors that affect the microscopic distribution characteristics of capillary imbibition.