The invention discloses a preparation method of high-concentration high-color-fastness masterbatch for fibers. The method comprises the following steps: 1) weighing the following raw materials by weight: 8-30 parts of PET, 0-50 parts of PBT, 0-50 parts of pigment, 0.3-1.5 parts of a dispersing agent and 0.5-1.2 parts of a rheology modifier, with the intrinsic viscosity of PET being 0.66-0.82 dL/gand the intrinsic viscosity of the PBT being 0.82-1.0 dL/g; (2) after the raw materials are weighed according to the proportion, adding the weighed raw materials into a mixer, stirring the materials for 10 minutes to fully mix the materials, then extruding the uniformly mixed materials in a double-screw extruder, and carrying out water-cooling, strip drawing and pelletizing to obtain the masterbatch. The double-screw extruder is provided with a vacuumizing device. The masterbatch prepared by the invention is suitable for high-color-fastness PET fibers, can solve the problems of poor thermal migration resistance, poor washing fastness and spinnability of the existing colored fibers before spinning, improves the phenomena of yarn drifting and end breaking in the fiber preparation process, and is beneficial to prolonging the service cycle of a spinning assembly.