The invention relates to an in-situ luminous flux monitoring and exposure dose compensating method. The method comprises the following steps: measuring the luminous flux of a main light beam going through an exit slit and entering a vacuum chamber before a lithography experiment by using a photodiode, monitoring the intensity of main light beam going through the exit slit and entering the vacuum chamber and the luminous flux of stray lights directly proportional to the main light beam during experimental exposure, and adjusting the exposure time of each lithographic area in real time accordingto the change of the intensity of the stray lights to compensate for the exposure dose in order to keep the exposure dose constant in the XIL large-area exposure pattern splicing process, ensure thatthe nanostructure in the finally obtained large-area exposure pattern is uniform and effectively improve the performances of a device formed by the exposure pattern.