The invention provides an experimental device for evaporating and post-evaporating thermotechnical hydraulic characteristics during in-tube flow boiling and an evaporation judging method. The device comprises a vertical tube experimental section, an electric heater and a control system thereof, a heating electrode and a data acquisition system. The device is characterized in that electrodes are separately mounted on the upper and lower ends of the vertical tube experimental section, the electrodes are connected to the electric heater, a heat resistor is mounted on the outer wall of the vertical tube experimental section, a pressure sensor and an optical fiber probe are mounted in the vertical tube experimental section, measuring holes are formed in the vertical tube experimental section in a spaced manner, a pressure difference transmitter is mounted between two adjacent measuring holes, and the heat resistor, the pressure sensor, the optical fiber probe and the pressure different transmitters are connected to the data acquisition system. By pre-judging an evaporating position to perform an experiment to obtain related data, experiment researches on evaporating and post-evaporating thermotechnical hydraulic characteristics in an in-tube flow boiling process are realized.