In order to solve the problem that the efficiency and precision are low if air situation is displayed and reported by grid coordinates(nine-nine and five-five grid), the typical application of the Gauss-Kruger projection in air defense command and control system is studied. The station's core horizontal orthogonal coordinates converting model can be founded by transforming the geography coordinates to Gauss-Kruger coordinates, working out the meridian convergence angle and setting up converting coordinates matrix. And then by calculating the educed unitary quadratic equation, the target station vertical coordinates are worked out. The Analysis shows that the model can be used to avoid complex coordinate's transformation computation, obtain higher precision than before, and by using which, Gauss-Kruger projection can be applied to describing air situation, displaying the firing command and converting the station's core horizontal orthogonal coordinates.