Abstract: In order to improve the utilization efficiency of sludge, the low temperature thermal-alkaline (LTTA) method was used to pretreat the sludge. The effects sI udge before and after modification with LTTA method on apparent viscosity of coal-sludge slurry (CSS) , rheological behavior and hysteresis loop area were studied in t he paper.The results showed that apparent viscosity of CSS was reduced effectively by LTTA method.When the shear rate was 100 s-1, apparent viscosity of raw CSS was 1 667 m Pa.s, after the sludge was modified by 90 °C-Ca (OH) 2for 1 h, 4 h and 24 h, the apparent viscosity of CSS decreased to 1 027.6, 698.0, 455.8 m Pa.s res pectively.While it decreased to 674.8, 436.3, 385.8 m Pa's after the sludge was modified by 90 °C-NaOH for 1 h, 4 h and 24 h respectively.At the same time, the rheolog ical behavior of the CSS was gradually transformed from pseudoplastic to Newtonian flow, and the shearing hysteresis of the CSS decreased, the hysteresis loop area of raw CSS was 2 078.0 Pa/s.Wheareas, the hysteresis loop area of raw CSS decreased to 774.5 Pa/s and 664.5 Pa/s after the sludge was modified by 90 °C-Ca (OH) _2, 90 °C-Na OH respectively for 1 h.The above results identified the LTTA method could efficiently improved slurrying ability and the rheological properties of CSS.