Abstract: Flat fan nozzle is the key component in roll cooling system, but the researches on the velocity and flow distribution of flat fan nozzle jets are not enough yet. Based on the theoretical studies on nozzles, a mathematical model of the jet geometric shape is established. The jet boundary is calculated and the flow distribution of flat fan nozzle is also obtained by using the fully formed circle free jet theory. Taking the GYQ 1981 B1 nozzle as an example, the calculations for the jet are done in detail. A test rig was developed, the jet performance of this nozzle is experimented by this rig, and the experimental results are found to fit the calculated results well.