The sky background brightness is measured with sky background measurement equipment in northwest region. The contour line of all-sky background brightness is given, and the spectrum distributing of the background brightness is analyzed. Then the corresponding brightness is simulated with the MODTRAN software under same weather situations, and the simulated result is analyzed. The result indicates that the background spectrum distribution changes slow relative to the theory, the detection ability can be improved through the narrowband filtering, and the simulated brightness basically agrees with the northwest region measurement data away the sun, and the error decreases with the decrease of zenith angle, but it is different when near the sun.