A model of post-craniectomy intracranial pressure dynamics is proposed in this article. Defining the craniectomy distensible volume the original Monro-Kellie principle is generalized. A craniectomy compartment is added to traditional intracranial system including blood, cerebrospinal fluid, and brain parenchyma. The system equation of generalized Monro-Kellie principle is solved with 4th order runge-kutta method. Volume of the new compartment is calculated with deflection solution. The model verifies that abnormal morphology of intracranial pressure (systolic value-21mmHg and diastolic value-13mmHg) in hypertension can be reduced to a normal range (systolic value-14.5mmHg and diastolic value-13mmHg) with decompressive craniectomy. Additionally the ICP-DC Size curve provides an effective interval (about 80-200 square centimeters) of craniectomy size for practice of decompressive craniectomy.