The invention relates to the field of large-sized barrel-type centrifugal compressors, in particular to a horizontal aligning method for a rotor and a partition plate bundle of a large-sized barrel-type centrifugal compressor. The horizontal aligning method comprises the following steps: perpendicularly fixing the partition plate bundle in the horizontal plane by utilizing a partition plate bundleadjusting tool; axially and horizontally fixing the rotor in the horizontal plane by utilizing rotor adjusting tools; adjusting the levelness of the partition plate bundle and the radial position ofthe rotor; and horizontally aligning the rotor and the partition plate bundle by utilizing a metering tool, wherein the metering tool comprises a dial gauge. According to the horizontal aligning method provided by the invention,the horizontal aligning of the partition plate bundle can be realized through a partition plate bundle adjusting tool; the radial position of the rotor can be adjusted through the rotor adjusting tools; and compared with a conventional aligning method, the horizontal aligning method has the advantage that the aligning precision can be improved by adjusting the levelnessof the partition plate bundle and the radial position of the rotor.