The invention relates to a method for decolouring a heavy alkylbenzene rectifying still bottom product. The method comprises the following steps of: adding the heavy alkylbenzene rectifying still bottom product and an adsorbent into a predeclouring tank; at the stirring rotating speed of 125 to 500 r/min and in the temperature range of between 50 and 100 DEG C, after oil stays in the decolouring tank for 30 to 60 minutes, allowing the oil to enter a plate-and-frame filter press to filter; after filtering, allowing the oil with international standardization organization (ISO) chroma less than or equal to 15 to enter an intermediate storage tank; when the ISO chroma is more than 15, performing secondary decolouring; filtering the decoloured oil by using the plate-and-frame filter press; allowing the decoloured oil to directly enter the intermediate storage tank; recycling the obtained adsorbent after being filtered for performing primary adsorption decolouring; mixing the oil in the intermediate storage tank and allowing the oil to flow out; mixing the oil and a catalyst Raney Ni in a metering mixer; inputting the mixture into a shell-and-tube hydrogenation reactor by using a high-pressure plunger-type pump to perform hydrogenation and decolouring; and filtering the mixture by using the plate-and-frame filter press and separating the catalyst to obtain the heavy alkylbenzene rectifying still bottom product, namely, heavy alkylbenzene with the ISO color number less than 7.0. Due to the adoption of the method, the high-chroma heavy alkylbenzene rectifying still bottom product can be processed, so that the ISO color number of the heavy alkylbenzene rectifying still bottom product is less than 7.0.