Nanoporous LiFe0.75Mn0.25PO4/C composites with different carbon contents are synthesized by the sol-gel method. The influences of carbon content on particle size, pore size distribution and cycling performance are studied. When the carbon content is 10.97 %, the primary particle size is tens of nanometers, the specific area is 90.06 m2·g−1, and the material is mainly mesoporous with a mean pore diameter of 9.17 nm. The initial discharge specific capacity of the material is 115.69 mAh·g−1 at 10 C, and it is 80.71 mAh·g−1 after 600 cycles. The good electrochemical performance of the material is attributed to the nanoparticle size, conductive carbon coating, large diameter mesopore size and high specific surface area.