The spread spectrum based flow watermarking, which can be used to trace anonymity abuses effectively, applies spread spectrum technique to encode watermark signals and embeds them into suspect flows. This serves to confirm the communication relationship among network users. The implementation of watermarking can be divided into four phases: Signal encoding, flow modulation, flow demodulation and signal decoding. It is important to choose the right watermark carrier that determines the robustness and invisibility of watermarking techniques. Since most applications using anonymous communication, such as Web browsing, instant message and remote login generate interactive traffic with unstable traffic rate, existing spread spectrum based flow watermarking adopting traffic rate as its carrier has big limitations. Furthermore, there exist some attacks against the invisibility of this watermarking technique, destroying the traceback effect. Based on the spread spectrum flow marking model, this paper proposes a novel flow watermarking technique that adopts interval centroid as its watermark carrier, which is insensitive to different types of flows. The theoretical analysis and experimental results show that this flow watermarking technique is appropriate for both interactive and non-interactive traffic, and can resist most existing attacks against flow watermarking.