PTMSP/PDMS-PEI composite membrane was prepared by solution casting method. To investigate the characteristics of this membrane, the analytical methods such as FT-IR, $^1$H-NMR, DSC, TGA, GPC, and SEM have been utilized. The number-average((equation omitted)) and weight-average((equation omitted)) molecular weight of PTMSP/PDMS copolymer were 501,516 and 675,560 respectively. The separation of the gas mixture($H_2$/$N_2$) through the composite membrane was studied as a function of pressure. The separation factor($alpha$, $eta$, (equation omitted)) of the composite membrane used in this work increased as the pressure of permeation cell increased. The real separation factor($alpha$), head separation factor($eta$), and tail separation factor ((equation omitted)) of PTMSP/PDMS-PEI composite membrane were 21.50, 49.14 and 1.84 respectively at $Delta$P 345.55 kPa and $25^{circ}C$.