The invention provides a slack finishing method for fabric on a mercerizing machine. The slack finishing method comprises processes of open-width fabric feeding, mercerization, fabric tightening roller loading, stacking, light alkali spraying, rinsing, neutralization, drying and cropping in sequence. The slack finishing method is characterized in that the humidity for open-width fabric feeding ranges from 15% to 20%; the padder pressure during mercerization ranges from 0.15 Mpa to 0.3 Mpa; the water pick-up rate ranges from 70%-80%; fabric tightening rollers adopt automatic rotation; the fabric is stacked for 10-20 min in a J-type box after mercerization; the tension of a fabric guide roller of a rinsing bath ranges from 160 N to 180 N during rinsing and neutralization; and the PH of the fabric after neutralization ranges from 6 to 7. According to the slack finishing method for the fabric on the mercerizing machine, by aid of low-pressure regulation of a mercerization tank, higher alkali containing quantity is maintained for the fabric so as to facilitate fabric shrinkage; stretching quantity of the fabric cover on a drum is reduced through automatic rotation of the fabric tightening rollers, and additionally, stacking of the fabric after mercerization facilitates warp-wise shrinkage of the fabric, so that the fabric has higher elasticity.