At present, the elastoplastic statistical and poroelastic models are often used to explain the mechanism of wellbore instability.  The effect of poroelastic dynamics induced by the wellbore excavation in tight formations in the early times is ignored. In this paper the transient model of wellbore instability in a porous formation with an anisotropic stress field was established. A Fourier transform technique was employed to decompose the governing equation for the 2D heterogeneous stress field into two one-dimensional problems. Meanwhile, the numerical convergence criteria were analyzed, and the effect of poroelastic dimensionless parameters on the transient failure was also included in the modelling process. The results show that: the pore pressure field, effective stress field, tangential stress field and radial stress field near the wellbore calculated by the new model are larger than those calculated by the traditional poroelastic model. Furthermore, shear failure is more likely to occur along the direction of minimum in-situ stress and the failure area increases with time. The fluid saturation in the porous media has the greatest influence on the transient wellbore instability. The wellbore tends be more likely to have higher instability with a higher fluid saturation. The new model established in this paper provides a new method for the wellbore instability mechanism analysis of tight formations and the evaluation of wellbore hydrostatic pressure. Also, it is significant for the management of wellbore stability in tight formations with an extra low permeability.