The utility model discloses a farmland non-point source nitrogen and phosphorus loss on-line monitoring device in the technical field of nitrogen and phosphorus loss monitoring. Cart, trolley top installation detection box, a soil placing box and a drainage pipe are respectively mounted at upper and lower ends of the left outer wall of the detection box; a storage battery is mounted on the right outer wall of the detection box; a control device, an observation window and a nitrogen and phosphorus content detector are respectively arranged on the front end surface of the detection box from leftto right; stirring blades are uniformly mounted on the outer wall of the rotating rod; a detection probe electrically connected with the nitrogen and phosphorus content detector is mounted on the inner wall of the right side of the detection box; the device is simple to operate, the detection box can be pushed to a required position by utilizing the cart; the detection range and convenience are improved, to-be-detected liquid in the detection box is stirred and turned over, the accuracy of data detected and collected by the detection probe is improved, the single-chip microcomputer sends outsignals to control the alarm module to send out alarm signals of different levels through the circuit control module, and detection personnel can know the nitrogen and phosphorus loss condition in farmland soil in time conveniently.