The power industry is the pillar and foundation of the national economy. Among various power cables, XLPE insulated power cables have become a major variety of high-voltage power cables and have received widespread attention in recent years because of excellent electrical properties, good mechanical properties, good heat resistance, and simple structures. The maximum long-term operating temperature is 90℃. Once the temperature is exceeded for a long time, the aging will be accelerated and the life will be greatly reduced. It is found through testing that if the cables overheat, ethylene gas will be emitted. Therefore, if the ethylene gas sensor is used, the cable status can be effectively monitored in a timely manner and faults can be found to ensure the safety and long-term stability of the cable. This paper mainly studies the hydrothermal synthesis of nano-flower NiO and its detection of ethylene gas generated during the operation of XLPE power cables. A simple nano-flower NiO was synthesized by a simple hydrothermal method. NiO is prepared as ethylene gas sensor, and the experimental results show that its gas sensing performance is better. From the analysis of the experimental results, it can be seen that the detection limit of nano flower-shaped NiO to ethylene is low and the response value is large, indicating that NiO can be used to prepare a sensor for detecting cable faults, which provides a direction for the fault detection of cross-linked polyethylene power cables in the future.