Enhanced oil recovery by imbibition（IEOR）is the key technology to effectively develop tight reservoirs with“three lows”characteristics. The fracturing fluid interacts with the rock and fluid in the reservoirs to produce an imbibition effect in the stages of reservoir fracturing，flowback，and subsequent development. Studying the imbibition mechanisms of steady and unsteady fracturing fluid has important guiding significance for field development. Therefore，the concepts of steady and unsteady imbibition are first put forward in this paper. On the basis of simulating high-temperature and high-pressure environments，the differences in imbibition characteristics of fracturing fluid under different conditions are quantitatively characterized by combining nuclear magnetic resonance（NMR）technology with physical simulation experiments. The results show that the scales of the steady imbibition effects of fracturing fluid are 0.01–51.52ms，and those of the unsteady imbibition effects of fracturing fluid are 0.01–27.75 ms. In the early stage of imbibition，the imbibition rates of both kinds of imbibition are the fastest，and the imbibition effects preferentially appear in small pores（0.01–1.00 ms）. As the reaction continues，they appear in mesopores（1.00–10.00 ms）and then in macropores （>10.00 ms）. The unsteady imbibition efficiency is higher than the steady imbibition efficiency，but the unsteady imbibition tends to be stable earlier. Small pores are the main contributors to the imbibition efficiency of fracturing fluid，and tend to be stable at first，followed by mesopores and macropores. Although the imbibition efficiency of double media is better than that of single media as a whole，the imbibition stability time of each pore throat is relatively lagging. The unsteady imbibition efficiency is positively correlated with reservoir permeability and reservoir quality indicator. As permeability and reservoir quality improve，the contribution of mesopores to imbibition gradually increases，and the mesopores gradually replace dominant small pores and finally occupy a leading position.