Compacted graphite iron (CGI) is the preferred material for engines at present. However, in the process of intermittent cutting CGI, the tool is prone to produce thermal cracks, resulting in tool chipping failure. The initiation of thermal crack must be inhibited to solve this problem fundamentally. Therefore, a new idea of designing oblique gradient tool was presented, including selecting appropriate gradient tool material and designing accurate gradient direction angle. According to the proposed design criteria of the tool, five layers of WC-Al2O3 gradient tool materials were designed, so that the thermal conductivity and thermal expansion coefficient of gradient materials were increased along the direction of thermal crack propagation, thus effectively alleviating the concentration of thermal stress. According to the thermal crack initiation area of the tool for milling CGI, the gradient direction angle of the tool was accurately designed to be 71.6°. The accurately designed oblique gradient tool can inhibit the initiation of thermal cracks by reducing thermal stress concentration on the tool, thus preventing the tool breakage failure. The new designed oblique gradient cemented carbide tool TDA-X with excellent cutting performance was successfully fabricated, and the inhibition effect of TDA-X on thermal crack initiation was verified by experiment. The result showed that TDA-X remarkably inhibited the initiation of thermal cracks in the tool much better than the non-oblique gradient tool TDA and the commercial coated carbide tool, thus avoiding the tool chipping, greatly prolonging the tool life and ensuring the quality of machining surface. The successful fabrication of this tool was of great significance to the efficient machining of CGI, and opened up a new idea for the research and development of tools with high cutting performance.