The invention belongs to the field of solar cells and particularly relates to a multi-junction GaAs thin-film solar cell based on forward mismatched epitaxial growth. The solar cell comprises a GaAs cap layer, an In<x>(Al<y>Ga<1-y>)<1-c>As gradually-changing layer, a first knot In<x>Ga<1-x>As cell, a first tunnel junction knot, an In<x>(Al<y>Ga<1-y>)<1-x>As gradually-changing layer, a second knot In<x>Ga<1-x>As cell, a second tunnel junction knot, an In<x>(Al<y>Ga<1-y>)<1-c>As gradually-changing layer, a third GaAs cell, a third tunnel junction knot, a fourth knot GaInP cell, and a GaAs cap layer. The solar cell is prepared by using the forward mismatched epitaxial growth technology and the corroded substrate stripping technology. The stripped GaAs substrate can be reused for the epitaxial growth of the solar cell by the chemical-mechanical polishing, and cleaning technology, thereby achieving reuse of the GaAs substrate. Via the forward epitaxial growth technology, micro-area defects alternatively generated and controlled via lattice mismatching are gradually annihilated or extend toward edges, thereby ensuring well epitaxial growth of the cell and further improving photoelectric conversion efficiency of the solar cell.