Carbon deposition of organic heat transfer heater is a key factor for the fire risk assessment. In this paper, ultrasonic guided wave was used for the carbon layer detection of organic heat transfer heater. Also, the detection principle and its test system were described. However, due to multi-mode and dispersive nature of ultrasonic guided waves in pipes, time-frequency analysis was used for the identification of ultrasonic guided wave modes for carbon layer inspection. Time-frequency representation of received signal is closed to theoretical dispersion curve of L(0,2) mode. Furthermore, the relative error of experimental group velocity of received signals and theoretical group velocity is only 1.88%~3.48% by time difference method. Therefore, received signals mainly consist of L(0,2) mode. The research results provide a significant theoretical foundation for the carbon layer detection technology of the organic heat transfer heater.（蓝色部分为修改意见1的修改，英文摘要已经重写，实验部分为过去式，实验结果为一般现在时，做到简短、明了和达意）