This paper proposes a trajectory privacy measure for Silent Cascade, which is a prevalent trajectory privacy preserving method in LBS (location-based services). In this measure, the user’s trajectory is modeled as a weighted undirected graph, and the user’s trajectory privacy level is calculated through the use of information entropy. It is pointed out in literatures that any privacy preserving methods will be subject to privacy threats once the attacker has new background knowledge. Therefore, adversarial background knowledge is hierarchically integrated into this measure. The privacy metric result composes of the assumptive background knowledge and the corresponding trajectory privacy level. (K_UL(K_i+,K_i-),K_L(K_i+,K_i-)) association rules is also proposed to describe the assumptive background knowledge. Simulation results show that this metric is an effective and valuable tool for mobile users and the designers of trajectory privacy preserving methods to measure the user’s trajectory privacy level correctly, even the attacker has variable background knowledge.