Precision forging blade of aero-engine is a typical thin-walled part with complex curved surface. In order to improve the assembly accuracy of tenon fixture, a multi-point flexible fixture design method is proposed. Combined with the designed fixture structure, the assembly accuracy is evaluated and optimized. Firstly, based on the six-point positioning principle, the influence of milling force on the deformation of parts under different clamping positions was investigated, and then clamping position and clamping mode of the blades were optimized. Secondly, a coordinate measuring machine is used to test the assembly accuracy, revealing poor contact between the blade and blade basin positioning pillars/air inlet edge positioning pillars, with a maximum planar error of 0.0353 mm. Finally, an integrated processing strategy is implemented to optimize the assembly process of the tooling fixture. According to the test results of fixture assembly accuracy, when using the optimized tooling fixture process, assembly accuracy for the three adjacent planes of the blade tenon is ±0.0049 mm, ±0.0063 mm and ±0.0063 mm, meeting the processing requirements for the blade tenon.