The surface modification of diamond particles was performed by chemical plating of Ni on the surface of diamond. The phase analysis of Fe-based amorphous alloy was conducted using an X-ray diffractometer (XRD), the characteristic temperature points of Fe-based amorphous powder were tested using a differential scanning calorimeter (DSC), the interface bonding situation and the microstructure of interface products were observed using a scanning electron microscope (SEM), the element analysis of materials was performed using an energy-dispersive spectrometer (EDS), and the thermal diffusivity of composites was tested using a laser flash apparatus (LFA 447). The results showed that fully utilizing the in-situ exothermic reaction during the phase transformation of Fe-based amorphous alloy improved the flowability of Al liquid and enhanced the interface bonding capability of diamond/Al composite. The nucleation and growth of multi-component crystals after the phase transformation of Fe-based amorphous alloy increased the interface thermal resistance and reduced the peak thermal conductivity of the diamond/Al composite. The introduction of Fe-based amorphous alloy improved the thermal stability of the diamond/Al composite, increasing the thermal stability by 10.3% within the tested temperature range. Excessive Fe-based amorphous alloy or prolonged preparation time could cause carbonization of diamond particles.