Pb composition in Sn-Pb melting alloy is an important quality parameter in Sn refining process, but so far its on-line measuring has not been resolved. In this paper, an instrument of on-line measuring composition in Sn-Pb melting alloy was researched by adopting advanced ARM9 embedded platform, WinCE operating system and Ethernet communication technology. Tests show that its measuring range is 0~50%Pb and its measuring precision may meet industrial application requisition. The instrument can also communicate with PC by Ethernet interface and realize its data store, inquiry and Web publish based on SQL Server 2000.