The detection of nitrite ion concentration is an important parameter of water quality testing. The nitrite ion concentration detection system based on Ethernet is designed to realise accurate and remote measurement of nitrite ion concentration. The intelligent detection node consists of electrochemical three-electrode sensor, optimizing design of constant potential detection circuit, MCU MSP430 processor and SD card to realize accurate measurement and data storage. On system Gateway which is an Ethernet gateway with the core of ARM9 processor, typical data network Ethernet is used to construct intelligent nodes network, the embedded Web server is constructed successfully with transplanting TCP/IP protocol stack on the real-time operating system LINUX, so nitrite ion concentration on-line monitoring and reliable storage and remote transmission is realized.