Abstract: The formula of drug protein binding rate derived by Zeng is modified in this paper.According to the principle of equilibrium dialysis ,the modified formula of drug protein binding rate is as follows:where,W is the water of percentage(%)in plasma;,Dand PD are the concentration(g/L)of protein ,free drug and protein-bound,respectively;β_p is the apparent maxium binding capacity ofprotein(μmol/g);K_dp is the dissociation constant of drug-protein complex(μmol/L).