Multistage fractured horizontal well is the one of the key techniques that has been widely used in the development of tight oil reservoirs at present. Because the pore throat of the tight oil reservoir is at nanoscale and the reservoir has extremely low permeability，the unsteady crossflow in the rock matrix cannot be ignored. Therefore，a five-linear flow mathematical model is proposed，in which the threshold pressure gradient，the stress sensitivity and the unsteady crossflow in unstimulated area are taken into account. Laplace transformation，Pedrosa’s transformation and Perturbation transformation are applied to solve the mathematical model，and the bottomhole pressure at the Laplace Space is obtained，and the dynamic pressure curves are plotted in double logarithmic coordinates by Stehfest numerical inversion. The results show that the dynamic pressure curves can be divided into six flow stages and fit well with the field data，which verifies the model. Meanwhile，the crossflow coefficient，the elastic storativity ratio，the dimensionless permeability modulus of main fractures，the threshold pressure gradient，and the permeability of unstimulated reservoir are analyzed，so that the effects of these parameters on well testing curves are clarified.