The invention provides an immersion on-line multiphase measuring instrument and a measuring method. The measuring instrument comprises a packaging pipe, a window, an LED lamp, a modulable light source, a telecentric lens, an image sensor, a controller, a signal processing and outputting system and a display system, wherein the modulable light source comprises a power supply, a signal generator and an oscilloscope; the LED lamp, the telecentric lens and the image sensor are located in the packaging pipe, and the exposure period of the image sensor is less than the pulse period of the signal generator. A shooting probe used in the measuring instrument is small in size, movable and not prone to influence by fluid temperature and surrounding environments, so the measuring instrument is applicable to real-time, on-line and quantitative measurement of two-phase, three-phase or even multi-phase transparent and opaque systems; and the measuring instrument can capture high-contrast images of flowing of local fluids in a multiphase reactor, and the concentration, granularity and velocity distribution of bubbles, liquid drops, solid particles or the like in multiphase flow flowing at a high speed can be obtained by using the corresponding measure method and professional image processing software.