Influence factors of compactness of DF laser with cooling gas ejector were analyzed. Ratio of Laser Power to Gas Source Volume(RPGV) was chosen as DF laser compactness evaluation index and was calculated at different conditions. According to the result, RPGV increases at first, and then decreases with increasing nozzle array mass flux. RPGV gets maximum value when nozzle array mass flux is about 3.3 g·s-1·cm-2. In the case of same laser effluent parameters, increasing ejector driving gas total pressure leads to a bigger RPGV, but its effect on RPGV gradually becomes weak. Compared with helium as ejector driving gas, adopting nitrogen and air as ejector driving gas leads to higher RPGV.