Provided are an immersion-type online multiphase measuring instrument and method. The instrument includes a package tube; a viewport; LED lamps and a brightness-adjustable light source system including a power supply, a signal generator and an oscilloscope; a telecentric lens and an image sensor; a controller; a signal processing and outputting system; and a display system. The LED lamps, the telecentric lens and the image sensor are located in the package tube. The exposure period of the image sensor is less than the pulse period of the signal generator. The photographic probe used in this measuring instrument has the advantages of online quantitative measurement, small size, portability, less-impact of fluid temperature and the surrounding environment, adaptability to transparent and opaque reactors with two-phase, three-phase and more than three-phase.