The invention discloses a side insert of a roller shell of a high-pressure roller mill and a replacing method. The side insert is circle-shaped and is inserted in annular grooves respectively disposed at two side ends of the external surface of the roller shell along a circumference direction, and each annular groove is fit with radial section shape of the side insert; the side insert has a circle-shaped structure formed by splicing a plurality of inserts, and the adjacent inserts are bonded tightly by a bonding agent; and when the side insert needs to be replaced, the side insert is heated and the bonding agent loses viscosity, then an old side insert is replaced by a new side insert. The side insert has the advantages of being high in hardness, good in wear resistance, convenient to replace and long in service life, and also the side insert helps to alleviate wearing speed of a side shield and a lower side plate by giving indirect protection, and helps to prolong service life of the high-pressure roller mill.