Abstract: The principle of measuring night-time total atmospheric transmittance with the calibration method of Langley-plot is introduced. Accordingly, a measurement system consisting mainly of a telescope, an intensified CCD, and an imaging data acquiring card has been developed to measure the weak stellar irradiance from level 1 to level 6 during the spectral band of 350~750 nm. The results obtained after calibration have been compared with the simulated value of MODTRAN, which verifies the feasibility of the system’s design.