In order to gain insight on the topology characteristics of P2P IPTV systems and their online user behavior characteristics, This paper develop and deploys a multiprotocol P2P IPTV network crawler, called TVCrawler, which enables users to launch an in-depth measurement and comparative research for several well-known P2P IPTV systems, including PPLive, PPStream, and UUSee. This paper presents results from experiments and research efforts on these large-scale P2P IPTV overlay graphs. Major findings include 1) more than 50% of online users are unreachable because they lie behind NAT or firewall; 2) fluctuation range of churn increases with the increase of the population of channel, and there exists a power-law upper bound for fluctuation range of churn; 3) session length of peer follows a stretched exponential distribution; 4) while the in-degree of PPLive follows the power-law distribution with exponential cutoffs, the in-degree of PPStream appears to have multiple separate power-law regimes with different exponents, and the in-degree of UUSee approaches Weibull distribution. 5) All of measured P2P IPTV overlay graphs appear as a disassortative mixing; 6) small-world networks are ubiquitous in measured P2P IPTV systems. 7) the PPLive overlay graph is clustering, but the others are not; 8) all of measured P2P IPTV overlay graphs have robustness similar to that of a power-law graph, but PPLive is more vulnerable to malicious attacks and more robust to random failures than the other. These research and findings Will be good for designing and optimizing of P2P IPTV protocols, but also for monitoring, directing, and dominating the P2P IPTV system.