Due to the use of staged or volume fracturing technology in the area of ultra-low permeability or tight reservoirs，the area around the horizontal well will form a complex fracturing network and the oil reservoirs are divided into matrix blocks with different shapes，and the fracturing fluid or water will contact with the matrix blocks in a large area when flowing in fractures. Dynamic counter current imbibition will occur and is dominant in the mass transfer process between matrix and fracture under the capillary force. In order to simulate dynamic counter current imbibition properly，the radical integration boundary element method（RIBEM）was used. A mathematical model suitable for the counter current imbibition was established. By comparing the result of RIBEM simulation with those of differential method, numerical calculation,experimen? tal data and analytical solution，the accuracy of the model was proved. This method is suitable for the calculation of cumulative recovery efficiency of a single matrix block with complex shape. Compared to traditional differential method，RIBEM does not need to divide network in the matrix block and the calculation process becomes easy，efficient and more suitable to the practice.