The present paper is a contribution to the phenomenological modelling of fatigue non-linear cumulative diffuse damage in short glass fibre reinforced thermoplastic matrix composites. In such materials, fatigue damage kinetic exhibits three stages, namely: (i) material softening and damage initiation, (ii) coalescence and propagation of micro-cracks and (iii) macroscopic cracks propagation and material failure. The proposed model is built in the framework of the continuum damage mechanics and aims at predicting these three stages of the damage evolution. This model is based on the approach initially proposed by Ladevèze and Le Dantec [Ladevèze P, Le Dantec E. Damage modelling of the elementary ply for laminated composites. Comp Sci Technol 1992;43:257-67]. It extends the previous approach and takes into account the important stiffness reduction observed during the first damage stage. The above is modelled by the integration of a combined Norton-like power law and an exponential law expressing the damage rates as a function of the associated thermodynamic dual forces. The model has been formulated in terms of strain energy, so that makes easy its numerical implementation into the finite element code Abaqus/Standard through a user defined material subroutine UMAT. Numerical simulations are performed on a short glass fibre reinforced thermoplastic described by a given set of damage parameters. Damage evolutions predicted by the developed model reproduce well those observed for this kind of composites under cyclic loading. A parametric study is performed to understand the effects of the model parameters on the damage accumulation and their sensitivity on its kinetic. The sensitivity study would be useful since it contributes to optimise the ongoing experimental procedure aimed at identifying the damage model parameters.