Abstract: The accuracy of predicting the formation pore pres- sure is the foundation of the wellbore stability analysis. Seismic data prediction before drilling is usually adopted in traditional prediction of formation pore pressure;howev- er,its accuracy can not meet the requirement of drilling operation.On the basis of the Grey Theory,a new meth- od of un-drilled formation under the bit is proposed,and then a grey model of formation pore pressure prediction while drilling is established.According to the drilling re- suits of the upper drilled section,prediction on the un- drilled formation pressure under the bit while drilling is carried out.Compared with dc index method and rock strength method,the grey model is characterized by high precision,simple design and easy calculation.The practi- cal application proves the modeling is a superior method on prediction.