By mixing PEG or LiCl respectively into the casting solution, the polyurethane (PU)/SiO2 hybrid membranes were prepared by means of immerged phase inversion process. The method of NMR relaxation time T1 value was used to determine effect of the segmental movement of PU macromolecule in the casting solution on the microporous structure of membrane. The membrane section photos of SEM and properties of the PU hybrid membranes were investigated respectively. The results showed that blending SiO2, and with LiCl or PEG into the PU solution system, the T1 value synchronistically decreased. That was relaxed of PU chain winding and ease to move. The additives of PEG could influence the casting solution of PU, it enhanced the distribution of SiO2 in the casting solution, and formed fine interfacial micropores, meanwhile the retention of the membranes were increased to certain degree whereas the pure water flux could kept at a higher level. Similarly, adding some quantity of LiCl could obtain the membrane with higher retention under the condition of slightly loss of pure water flux. Even thought that there were equivalent quantities of finger pores in section of membrane, mixing PEG or LiCl in the casting solution, the SEM section photos indicated that there were more micropores in the wall of pore.