A damage evolution law for porous phenolic composites is developed by using phenomenological point, which along with the improved Johnson-Cook model is then implemented into a finite difference code for one-dimensional strain wave problem. The velocity history of free-face particle is recorded and correlated with the simulated result to evaluate the damage parameters. Good agreement shows that the proposed damage evolution law and spall rule are feasible. Besides, the relationships between the striking velocity of flyer and the spall thickness as well as spall time are investigated. This study provides a methodology to quantify the damage evolution of phenolic composites under impact loading.