Tight oil and gas have become the key point of research and development in the field of unconventional oil and gas. As an important part of tight oil and gas seepage mechanism，the influence of imbibition can effectively improve the recovery of oil and gas. However，there are few researches on it. The imbibition experiment was conducted under different conditions on matrix cores of the domestic typical tight oil and gas reservoir. The phenomenon and results in this experiment were analyzed. A new seepage mechanism of the matrix cores in the tight oil and gas reservoir was purposed，called layer imbibition theory，which means the imbibition starts from the surface of the cores to the inner of the cores step by step.The imbibition efficiency formula according to the quality method was revised and the accurate results of the indoor imbibition experiment were calculated，and the results were calibrated to the actual formation scale. The results show that the oil/water imbibition efficiency of the matrix cores in the tight oil and gas reservoirs is 12%-18% under reservoir condition and imbibition speed is estimated in the order of magnitude of 1×10-4 cm/min. It is necessary to use the imbibition method together with other displacement methods for the tight oil and gas reservoirs so as to improve the final recovery of the tight oil and gas.