As one of the common capacity-building conductors, aluminum conductor composite core (ACCC) is widely used in overhead power transmission lines. However, its mandrel is vulnerable to damage in the construction process, which may lead to breakage of conductor strand and wire. Therefore it is urgent to study the nondestructive testing technique for ACCC in service. Because the ACCC is made of outer aluminum conductor and inner composite core, the traditional detection methods cannot effectively inspect the defects in the mandrel. This paper proposes a method to detect defects of in-service ACCC using a linear scanning computed tomography (CT). In this CT method, an X ray source and a detector are moved in a relatively parallel line around the fixed detection object, which has the advantages of simple structure and convenient installation. Furthermore, one and two linear CT scanning modes are designed according to different situations, and the corresponding principle experiment platform is built. The three common mandrel defects of fracture, split and crack were tested in small batches under laboratory conditions. The experimental results show that one relative linear scanning CT can be used for rapid detection of mandrel defects with large defects, and two relative linear scanning CT can be used for fine detection of small defects existing in mandrel. Compared to traditional X-ray transmission imaging detection, the method in this paper provides a new idea and reference for inspecting the internal defects of the ACCC.