Aiming at the problem of large error of temperature field in thermal field detection of internal combustion engine cylinder head, combined with the difference of thermal field of fire surface at different positions, a multi-scale division method of the fire surface based on gradient variation law was proposed, which laid a foundation for the accurate detection of the thermal field distribution of the flame face. Firstly, according to the heat transfer law of the cylinder head gas side and the temperature of the thermal field, the fire surface was divided into inward-exhaust, inward-intake, exhaust-valve nose bridge area and peripheral area with the fuel injection hole as the center. Secondly, the variation characteristics of temperature gradient in each region are analyzed, and each region is divided into different scales based on different gradient variations. Finally, the number and location of temperature measurement points in different scales were determined, and the temperature of each point was measured by combining thermocouple temperature measurement technology. The experimental results show that compared with the temperature measurement values under a single scale, the temperature measurement method based on multi-scale division idea can highlight the temperature variation in the nose bridge area, which is prone to thermal fatigue damage, and provides a more reliable data source for the detection and evaluation of thermal fatigue damage in the inner wall of cylinder head.