The microstrip antenna has the advantages of small size, light weight and low profile, can be designed as a radio altimeter antenna. The rectangular patches are composed of broadband and high gain antenna array, using the way of arraying. The increasing of the substrate thickness expand band and multi section impedance obtain large gains. This structure is applied to design the four element microstrip antenna array. The center frequency is 4.3 GHz, the bandwidth is more than 200 MHz. the method of equal amplitude parallel feed is adopted. According to HFSS the simulation results show that the maximum gain is 11.3 dB, the VSWR is less than 2, and the 3 dB beam width is about 46 degrees. The bandwidth is expanded to4.08~4.52 GHz, and the maximum gain is 10.33 dB. The structure of the antenna array is simple, and its good characteristics can be applied to the small aircraft altimeter system.