There are many kinds of aero-engines with different shapes and sizes. The traditional teaching programming rigid bolt tightening method is no longer suitable for small and medium-sized batch and multi-type engine bolt tightening due to its disadvantages of slow efficiency, poor safety and high programming threshold. A flexible tightening method of engine bolts based on digital twin was proposed, including bolt tightening scheme, digital twin simulation of bolt tightening process, consistency calibration of physical space and digital space, and so on. The kinematics model of the robot was established by D-H parameter method, and then the robot kinematics was solved. The trajectory of the robot was planned by quintic polynomial interpolation method. The offline program was generated by digital twin simulation, and then the coordinate deviation was eliminated by consistent calibration. Three sets of experiments were carried out by using off-line programming and teaching programming respectively. The results show that both off-line programming and teaching programming can complete the bolt tightening work of aero-engine, while the flexible bolt tightening method based on digital twin can quickly and flexibly tighten the bolts of aero-engine.