Abstract: In order to improve the performance of a 18-element linearly-polarized waveguide array antenna, three methods were used to optimize the array antenna, including changing the the feeding network's seal shape for the array unit and optimizing the feeding probe, improving the cell antenna, and reducing the space between two cell antenna. Then, theoretical analysis, numerical simulation and experimental measurement of this radial line helical array antenna were carried out. The results of simulation show that at the centric frequency of 1.57 GHz, the reflectance is 0.067, the gain is 18.73 dBi, and the axial ratio is -39.82 dB; in the frequency range from 1.43 to 1.76 GHz, the reflectance is below 0.15. The experimental measurement shows that at 1.57 GHz, the voltage standing wave ratio (VSWR) is 1.19 (the corresponding reflectance is 0.087), the gain is 17.65 dBi, and the axial ratio is -39.2 dB; in the frequency range from 1.47 to 1.77 GHz, the VSWR is below 1.4 (the corresponding reflectance is 0.17). Therefore, the simulation results agree with the experimental results well.