Abstract: Tungsten (W) coating with thickness of 300 nm was deposited on the diamond surface by magnetron sputtering, and the influence of heat treatment parameters on the phase transformation of the W coating on the diamond surface was studied. The W-coated diamond particles after heat treatment were then prepared into Cu matrix composites by pressure infiltration. The influence of the phase evolution of the W coatings on the thermal expansion coefficient of the Diamond/Cu composite was studied systematically. The results show that the W coatings on the diamond particles begin to react with the diamond to form W₂C at 900℃. As the heat treatment temperature increases, the C atoms in the diamond particles gradually react with W and W₂C to form WC phase. Noteworthy, the C atoms on the diamond surface tend to graphite at high temperature of 1200℃. The evolution of the tungsten-coating phase significantly influences the interface bonding of the diamond/Cu composites, and subsequently impacting its coefficient of thermal expansion. As the W phase gradually transforms into the corresponding carbide phases (W₂C, WC), the interface gap in the composite diminishes, leading to an increase in the effective volume fraction of diamond. Consequently, the coefficient of thermal expansion exhibits a fluctuating trend with initial decrease followed by an increase, consistent with variations predicted by Turner and Kerner models. In cases where the interface structure of the composites is Diamond/WC/W₂C/Cu, its coefficient of thermal expansion can be as low as 6.35×10⁻⁶ K⁻¹ (50℃). Good interface bonding and the low thermal expansion coefficient and high elastic modulus of the interface layer play a critical role in enhancing load transfer efficiency and reducing the coefficient of thermal expansion for the composite. This work provides a theoretical basis for optimizing the coefficient of thermal expansion of diamond/copper composite and the selection of the interface carbide layer.