Abstract: Sky background luminance is very important for space target detection and identification. In order to research on it, the measurement system of sky background luminance was introduced, compared with the sun photometer CE318 made by CIMEL Co. in French, a good consistency was achieved. On the time when a certain space target transited, the sky background luminance on the target orbit was measured by the system. The data was analyzed and studied. The primary variation characteristic of the sky background luminance on the target orbit was acquired. Important information is provided for application.