In order to clarify the relationship between the porosity of membrane and the kinetics parameter of membrane formation (D a ) in polysulfone (PSF) ultrafiltration membrane, phase-inversion process was used in PSF membrane preparation from PSF casting solution with poly(ethylene glycol) (PEG) or poly(vinyl pyrrolidone) (PVP) introduction. A new method used to calculate D a was founded and an equation of D a =d 2 /t was established on the basis of the relationship between wet membrane thickness and coagulation time. It was found that there was substantial relationship between D a and porosity in PSF ultrafiltration membrane, and that the linear relationship between D a and porosity in PSF ultrafiltration membrane could be described as P(%)=a+bD a .