An atmospheric radiation measuring instrument (DTL-1) was developed to research the sky brightness radiance in real time, which could measure the integral radiance at wavelength of 400 to 1 000 nm in two working modes of all-sky scanning and given direction sounding. The design scheme was introduced in detail by the functional-block diagrams of system structure and the flowchart of signal acquisition about DTL-1 and the main technical parameters of instrument were also presented. Then, the absolute integral radiance of the DTL-1 was calibrated in the spectral range of 400 to 1 000 nm by using a wide wave range absolute calibration method, and measuring stability under a constant brightness was tested. At last, the results measured by DTL-1 for practical data from different regions were compared to simulation of MODTRAN5. The results show that the average radiance of work stable test and the root-mean-square error are 36.496 W/m2·sr, and 0.463 W/m2·sr, respectively, and the variance between measurements of DTL-1 and simulations of MODTRAN is under 20%. It demonstrates that DTL-1 can satisfy the requirement of scientific research and actual application.