Low-density C/C composite was modified with Mo2C interlayers through molten salt method. The formation mechanism of Mo2C layers was analyzed; the effects of flux composition and reaction temperature on the abrication of Mo2C interlayers were studied; the influence of Mo2C-modification on the microstructure of C/C composite was also observed. Results show that the reaction between ammonium paramolybdate and C for the formation of Mo2C layers is mainly consist of three steps: decomposition from ammonium paramolybdate to MoO3, reduction of MoO3 to MoO2, carbonization of MoO2 to Mo2C. The appropriate flux of LiCl–KCl and the suitable reaction temperature as 1000℃ is determined according to the experimental results. Flower-like Mo2C layers are formed and then continuous Mo2C layer cover the surface of C phase with the increase of reaction time and temperature. The Mo2C/C interface showcases good bonding state. The pyrolytic carbon phase near the interface shows high order degree as the result of catalytic graphitization and stress graphitization. Modification of Mo2C interlayer for the C/C composite can facilitate the infiltration of Cu and its joining with Cu.