A special kind of astronomical fiber device which is called as optical fiber brush is introduced. Its fabricating technique and the supermode coupling characteristics are investigated. Due to the low dispersion of astronomical imaging, the supermode coupling charateristic of the optical fiber brush is studied. In order to investigate and compare the divergence angle of different supermodes, near-field and far-field distributions of them are also calculated. Comparing with the far-field mode radius, it′s found that the in-phase supermode has the least divergence angle and the best beam quality. The research result can be helpful to investigate simillar fiber mode converters.