While the source routing mechanism has a unique advantage on enhancing the coverage of IPv6 network topology discovery, it is considered to be inefficient in the current IPv6 network topology discovery technology. Research into the nature of source routing mechanism reveals the fact that the total number of targets to be probed grows rapidly during the progress of topology discovery. This means probing process has to face a huge probing space and this is the main reason for the inefficiency. This paper described how probing space expands, analyzed the reasons why probing space expands, then offered a solution to suppress this fast expansion to improve the efficiency of source routing mechanism. The experiment results showed that the improved solution suppressed more than 65% of the probing space expansion effectively on the premise of not affecting coverage rate.