The invention discloses a frequency modulation method for long blade assembly, relating to a frequency modulation method for blade assembly and aiming at solving the problem that the traditional whole-stage blade frequency modulation method is effective to first-order frequency modulation of a whole-stage blade and is ineffective to second-order frequency modulation of the whole-stage blade. The frequency modulation method disclosed by the invention comprises the following steps of: 1, determining geometric parameters of a shroud band and a lacing wire; 2, after assembly of the whole-stage blade is finished, carrying out first-order frequency F<1>d test on the whole-stage blade, wherein when the F<1>d is more than Kn and less than 1.03Kn, thickness or clearance needs to be reduced, and when the F<1>d is more than 0.94Kn and less than Kn, thickness or clearance needs to be increased; and 3, carrying out second-stage frequency F<2>d test on the whole-stage blade, wherein when the F<2>d is more than Kn and less than 1.03Kn, height of the lacing wire relative to the root of the blade and sizes of a long axis and a short axis of the lacing wire need to be increased, or clearance is reduced, and when the F<2>d is more than 0.94Kn and less than Kn, height of the lacing wire relative to the root of the blade and sizes of the long axis and the short axis of the lacing wire need to be reduced, or clearance needs to be increased. The frequency modulation method disclosed by the invention is used for assembly of whole-stage blade of a steam turbine.