A new idea of adjusting half-power beam width is applied to design a navigation antenna of Beidou-2 B3 band, which can realize beam width transformation and apply to different beam forming scenes. The microstrip patch array is modeled in HFSS simulation software. On the basis of keeping the right-handed circular polarization, the half-power beam width can be reduced or expanded without changing the structure of the array, the directional beam forming and wide beam forming with adjustable main lobe direction can be realized to cope with different working environments, and the directional anti-interference ability can be achieved. The simulation results show that the maximum gain of directional beam forming and wide beam forming at B3 center frequency is about 7.13 dBi and 3.56 dBi, the half-power beam width is 52° and 119° respectively, and the axial ratio width in xOz plane is 90° and 166° respectively. In B3 frequency band, the reflection coefficient of each feeding port is under -11 dB, and the isolation degree of adjacent ports is under -28 dB. The designed navigation antenna with adjustable beam forming mode is suitable for working scenes that often switch between open space without shielding and specific shielding environment, and it can improve the low gain of traditional mobile navigation terminal in the environment of high shielding angle such as “Canyon City”.