The invention provides an on-line runner descaling method of a centrifugal compressor for oilfield associated gas. The on-line runner descaling method comprises the following steps of 1) arranging a back washing pipeline; 2) performing pollution discharge treatment: closing the back washing pipeline, opening a pollution discharge pipeline, discharging impurities at the bottom of an outlet separator; 3) carrying out back washing detection: closing the pollution discharge pipeline, opening a detecting pipeline on the back washing pipeline, and checking whether impurities exist in condensate oilor not by means of a detection hole; and 4) carrying out back washing treatment: closing the pollution discharge pipeline and the detecting pipeline, opening the back washing pipeline, and washing a centrifugal compressor, wherein the washing time is 0.5-1 hour, and the washing frequency is less than 1 times every week. The back washing pipeline is directly arranged on an existing centrifugal compressor pressurizing process pipeline for oilfield associated gas, and the separated condensate oil enters a runner of the centrifugal compressor along with airflow through the back washing pipeline, and the washing and carrying effect of the condensate oil is much larger than the deposition effect, so that impurities accumulated in the runner are washed down and are entrained to the outside of a machine body.