A method and a system for preparing xylan and calcium carbonate, xylan, a cellulose fibre, and precipitated calcium carbonate. The method includes extracting xylan from cellulose fibres with sodium hydroxide to form a mixture containing cellulose fibres and an extract solution containing xylan； removing the cellulose fibres from the mixture to isolate the extract solution； adding carbon dioxide to the isolated extract solution to form a mixture containing a liquid brightener and precipitated xylan； separating the brightener from the mixture to isolate the precipitated xylan； adding calcium hydroxide to the separated brightener to form a mixture of precipitated calcium carbonate and liquid sodium hydroxide； and removing the sodium hydroxide from the mixture to isolate the calcium carbonate.
