The invention provides a sensor to measure surface active agent of water positive ions, which is composed of teflon outer sleeve, graphite inner sleeve and Ag/AgCl internal reference electrode. Active film is coated on inwall of graphite pipe. Internal reference solution is packaged between outer and inner sleeves and Ag/AgCl internal reference electrode is packaged in the solution. The inner sleeve connects with testing conductor pipe. It is characterized in that: set radial sampling pipe in inner sleeve. Samples enter from sampling pipe and flow through both ends of conductor pipe. In application of this T-branch flow active film sensor, the entry direction of water sample is vertical to active film. Water sample impacting active film avoids the situation that currently flow-electrode having on response to the tested substance. It has simple producing process, large contact area with water sample and high sensitivity. The testing process is quick and simple.