This study develops a novel balancing method named as Low-Speed Holo-Balancing (LSHB), which can balance the flexible rotor without test run at high speeds. The principle of LSHB is mainly based on the holospectrum technique with following specialties. Firstly, the vibration response of rotor is described by three-dimensional holospectrum using multi-sensor fusion, instead of information from a single sensor. Secondly, the variation rules of the first two modal components are studied through the information in run-up or run-down stages. Then we can deduce the vibration responses at high speed from that at low speed. Finally, the first two modal components can be corrected simultaneously at the speed below the first critical speed. The advantages of LSHB are that the safety of balancing increases because the rotor need not run at high speed or at critical speeds as the traditional modal balancing method do, also the number of test runs and corresponding wastages decrease. The effect of method was validated by the experiments on a flexible rotor test rig.