An apparatus for measuring brightness transmitted by a light-emitting display unit of a laser telescope and observed by the user's eyes comprises a focusing unit (11), a sensor unit (12), a processing unit (13) and a display unit (14). A method of measuring brightness includes the steps of: providing a constant color temperature to simulate the external light; providing a tool platform and locating the apparatus for measuring brightness and the laser telescope under test thereon; further providing a light-impermeable cover to cover the laser telescope and the apparatus for avoiding the external light interfering with the measurement of brightness; then turning on the laser telescope and enabling it to range; picking up the light of a reticle and a measured value transmitted from the light-emitting display unit by means of the apparatus for measuring brightness; and at last, displaying the measured value on the display unit of said apparatus for measuring brightness.