In order to reveal the solute existing form and its effect on the microstructure and mechanical properties in the supersaturated solid solute films, TiB_x/Al composite film with 7.1%TiB_x (atomic fraction) was synthesized by magnetron co-sputtering using Al and TiB₂ targets. The microstructure and mechanical properties of the film were characterized by 3D atom probe cambined with X-ray diffraction, transmission electron microscopy and nanoindentation. The results reveal that TiB_x compounds were largely dissolved in the Al lattice due to atomic mixing and sufficiently low mobility of the atoms during growth. The film grain was refined to nanoscale and amorphous gradually result from the severe lattice distortion. The film consisted of a two-phase structure in which lower solute content (about 2%TiB_x) nanocrystals dispersed in amorphous matrix with a higher solute content and gained high hardness. Combined with the experimental results, the microstructure formation of the composite films was discussed from thermodynamic and kinetic aspects. This study provided the direct experimental evidence of compound dissolved in highly supersaturated solid solution.