Fine-grained spectrum management is promising in addressing the contradiction between the access requirements for massive radio systems and the shortage of spectrum resources. It is difficult to identify the radio system and securely transmit its spectrum identity information due to the complexity, openness, and variability of the electromagnetic environment, which has become a major challenge for the fine-grained spectrum management. In this paper, we consider a practical and proactive spectrum monitoring scenario, where the spectrum monitoring node monitors the communication signal sent by a source node to a destination node for the fine-grained spectrum management. A malicious node eavesdrops on the transmission for accessing and deceiving them. We propose a secure transmission method of spectrum watermark for the fine-grained spectrum management. The spectrum watermark for identifying the radio system is hidden in the normal communication signal via adjusting the embedding parameters, and has no effect on the required transmission performance. We investigate the transmission performance of the spectrum watermark in two typical use cases. In first case, the transmission rate of the communication signal is given and fixed. In another case, the transmission rate of the communication signal can be adaptively adjusted according to the channel quality. The analysis and simulation results show that the proposed method achieves the optimal and secure transmission of the spectrum watermark by adjusting the embedding parameters of the spectrum watermark while ensuring the required and optimal transmission performance of the communication signal. INDEX TERMS Fine-grained spectrum management, spectrum watermark, embedding parameters of spectrum watermark, secure transmission. I. INTRODUCTION With the rapid development and broad application of wireless communication technology, the number of mobile users and their requirements for bandwidth are growing rapidly. According to Cisco's 2019 visual network index, global mobile data traffic would grow 7-fold from 2017 to 2022 [1]. The shortage of spectrum resources has become a bottleneck The associate editor coordinating the review of this manuscript and approving it for publication was Ruofei Ma. restricting the sustainable development and application of wireless communication technology. Fine-grained spectrum management, which can improve the effectiveness of spectrum management, is regarded as one of the most promising methods to solve the contradiction between the increasing spectrum demand and the shortage of spectrum resources. Radio spectrum identification is critical to achieve the fine-grained spectrum management. Due to the openness of the electromagnetic environment and the boundless of wireless transmission, the spectrum identification information of