The human insulin gene modified with a C-peptide was synthesized according to the plant-preferred codon, and a fusion gene expression vector of insulin combined with green fluorescent protein (GFP) was constructed. The optimization of the flax callus culturing was undertaken, and a more efficient Agrobacterium-mediated genetic transformation of the flax hypocotyls was achieved. The critical concentration values of hygromycin on the flax hypocotyl development, as well as on its differentiated callus, were explored by the method of antibiotic gradient addition, and the application of antibiotic screening for the verification of positive calluses was assessed. The fusion gene of insulin and GFP was successfully inserted into the flax genome and expressed, as confirmed through polymerase chain reaction and Western blotting. In conclusion, we have established a flax callus culture system suitable for insulin expression. By optimizing the conditions of the flax callus induction, transformation, screening, and verification of a transgenic callus, we have provided an effective way to obtain insulin. Moreover, the herein-employed flax callus culture system could provide a feasible, cheap, and environmentally friendly platform for producing bioactive proteins.