Abstract: The mass transfer shape factor between matrix and fracture is one of the most important parameters in the numerical simulation research on porous flow in fractured reservoirs. For the purpose of indicating the nonlinear and nonsteady characteristics of mass transfer between fracture and matrix in low permeability fractured reservoirs, this paper firstly analyzed the inapplicability of pseudo-state shape factor and existing nonsteady shape factor used in low permeability fracture reservoirs, then the variable coefficient of apparent permeability was introduced based on the fitting analysis results of the v-!-abla p curve of 23 core samples, and then a nonlinear diffusivity equation was developed based on the variable coefficient to govern the mass transfer between fracture and low permeability matrix. Integral method and method of moments were applied to deduce semi-analytical solution of zero dimension nonlinear diffusivity equation at the earlier and later stages, to lead the nonsteady shape factor and the new mass transfer function. By comparison with the pseudo-state mass transfer model, which was restricted to describe the mass exchange in the systems whose pressure changes quite slowly, the new shape factor was effective to simulate the nonlinear and nonsteady mass transfer between matrix and fracture in low permeability fractured reservoirs, and the fine grid finite element numerical model verified the reliability and availability of the nonsteady shape factor and the new transfer function.