Abstract: Gel breaking experiments about four different acid solutions were conducted,the fracturing fluid viscosity, residue particle size distribution,residua content and sugar content in the gel breaking liquid were separately tested by rotational viscometer,Malvern laser particle size analyzer,centrifuge and spectrophotometer,and the breaker for the low temperature coal reservoir was optimized. The damage ratio of gel breaking liquid to the coal reservoir was evaluated by experimental tests. The results show that:Hydroxyropyl-guar gum fracturing fluids could be broken quickly and com- pletely at the temperature 20 degrees by using the HXM+NH4 Cl,the greater the concentration,the faster the gel break- ing speed;When the concentration is 0. 10% HXM+0. 05% NH4 Cl,the fracturing fluid viscosity can drop less than 5 mPa·s within 2 hours,the residue particle size is 86 μm,residue content is 70 mg / L. The damage ratios of perme- ability by coal samples are lower than 20% when the original permeability is less than 0. 87×10-15 m2 . Furthermore, the higher the original reservoirs permeability,the greater the damage ratio is. Compared with the oxide breaker,the advantages of the composite acid breaker are quite torn glue at low temperature,little residual content and the minor injury.