Sulfonated polyimides (SPIs) are extremely suitable as polymer electrolyte membranes (PEMs) for fuel cell applications, except for their poor water stability. Cross-linking is a method that is commonly used to improve the weak hydrolytic stability of SPI membranes. However, this strategy significantly decreases the proton conductivity of the membrane, which leads to a lower fuel cell power density. In this work, a cross-linked SPI membrane containing a highly branched polymer main chain was fabricated as a PEM. With a similar ion-exchange capacity value, the cross-linked membrane containing branched main chains showed an improved proton conductivity. Also, this membrane remained 92.3% of pristine weight after a hydrolytic stability test about 120 hours. In a single direct methanol fuel cell, the cross-linked membrane containing a branched structure showed a higher power density (53.4 mW cm(-2)) than the common cross-linked membrane (43.0 mW cm(-2)), indicating that branching is effective for improving the electrochemical properties of PEM-based cross-linked SPIs.