The invention relates to a continuous, synchronous and online monitoring method and instrument for the concentration and total quantity of TOC (total organic carbon), TN (total nitrogen) and TP (total phosphorus). The instrument mainly comprises an electromagnetic induction flowmeter, a sampling head, a reagent bottle, a carrier gas cylinder, an acidification reactor, an oxidation reactor, a semiconductor condenser, an ion absorber, a CO2 detector, a nitrate nitrogen detector, an orthophosphate detector and a data processor. The sampling head is connected with the acidification reactor through a sampling water pipe, a peristaltic pump and a flow sensor; ...; the CO2 detector, the nitrate nitrogen detector, the orthophosphate detector and the electromagnetic induction flowmeter are connected with the data processor by circuits. By means of the same set of sampling, acidification, oxidation, flow collection and data processing system and the three separate detectors, the concentration and total quantity of TOC, TN and TP in water can be measured continuously and synchronously. The instrument has the advantages of multifunction, compact structure, continuous data measurement, synchronism and instantaneity of measurement of different indexes, etc.