The invention discloses a system and a method for judging changing time of a brush roll in a cleaning section, and belongs to the technical field of rolling of cold-rolled plate strips. The method includes: winding a Pi tape on bristles of the brush roll to enable the Pi tape to be parallel to the roll diameter direction of the brush roll; pulling and holding the Pi tape to enable the Pi tape to wrap and compact the bristles of the brush roll; using the Pi tape to measure an operation side, a middle part and a driving side of the roll surface of the brush roll respectively so as to obtain an operation side value, a middle part value and a driving side value respectively; subjecting the operation side value, the middle part value and the driving side value to arithmetic averaging to calculate to obtain an average external diameter value of the brush roll; when the average external diameter value is smaller than a preset threshold value, changing the brush roll. Judgment is carried out by means of utilizing the Pi tape to measure the bristles of the brush roll and measuring multiple ranges to obtain the average external diameter value of the brush roll, so that service life of the brush roll can be prolonged greatly, waste on resources is reduced, steel coil production cost is lowered, and the roll is prevented from being changed blindly due to visual inspection.