Lithium iron phosphosilicates, i.e. Li1−xFe1+xP1−xSixO4, are novel cathode materials for lithium ion batteries. In this study, a facile one-pot method was employed to prepare Li1−xFe1+xP1−xSixO4/C nanocomposites. The dry gel, which contained all the starting materials, was calcined in an inert atmosphere to form the final product. The variation of their structural, morphological and electrochemical characteristics with x (x = 0, 0.15, 0.25, 0.5, 0.75 and 1) is studied. Single phase Li1−xFe1+xP1−xSixO4 with well-defined diffraction peaks can be obtained at an appropriate calcination temperature that varies with x. The phosphosilicate nanoparticles are embedded in an interconnected carbon network when the Li1−xFe1+xP1−xSixO4/C nanocomposites are obtained at 700°C. The carbon network is highly porous and electronically conductive, which is benefit to the deintercalation of lithium from the phosphosilicate particles. The discharge capacity of Li1−xFe1+xP1−xSixO4/C composite declines with an increase in x as well as calcination temperature. A discharge capacity of 70 mAh g−1 is preserved even if x is as high as 0.25 when the calcination temperature is 700°C. The particle growth and aggregation of Li1−xFe1+xP1−xSixO4 are severe when the calcination temperature is as high as 900°C, which leads to the collapse of carbon network. The interconnected carbon network is effective to increase the electrochemical performance of Li1−xFe1+xP1−xSixO4.