Double-root mandibular third molars are the common types of mandibular third molars that are prone to impaction. Based on the summary of clinical cases and experience, the commonly used crown-root separation technique is improved as diagonal root separation technique that divides the roots by the line connecting the distal axis angle to the root furcation. This new method improves the efficiency of the operation and reduces surgical trauma. This paper details the surgical method, minimally invasive mechanics, and key points of this technique, compares the technique with the crown-root separation technique, and discusses their joint applications.