Sulfonated polyimides (SPIs) have been shown to be promising materials for high temperature PEMs mainly because of their excellent mechanical and thermal properties as well as their chemical stability. In the present study, novel six-membered ring SPI copolymers were successfully synthesized. Hydrophilic soft segments of poly(ethylene oxide) (PEO) were copolymerized into the SPIs system to increase the water retention of the copolymers at high temperatures and low humidities. The relative ratio of the sulfonic acid-containing hard segments and the PEO-containing soft segments was controlled through variation of the molar ratio of 4,4'-diaminostilbene-2,2'-disulfonic acid (DSDSA) and diamine terminated poly(ethylene oxide) (PEO-diamine). Flexible, transparent, and mechanically strong free-standing membranes were successfully synthesized by the solution casting method. The novel soft segment-containing six-membered ring SPI copolymer membranes exhibited desirable thermal and hydrolytic stability. The conductivity measurements showed that the SPI copolymer membrane exhibited high proton conductivities, which were higher than those of Nafion® 115 at high RH levels (> 50%) at both 70oC and 120oC. The results from the fuel cell performance testing showed that this SPI copolymer membrane had similar open circuit voltage (OCV) and membrane conductance as Nafion 112® at 70oC and 80% RH. However, at 120oC and 50% RH, the novel SPI copolymer membrane showed a lower OCV (0.72 vs. 0.92 V), but much higher membrane conductance than Nafion® 112. As a result, the novel SPI copolymer membrane exhibited better fuel cell performance than Nafion® 112 when the current density was higher than 0.32 A/cm2.