Abstract: To study the stress-strain characteristics of the slurry in the main injection horizon of overburden,the experi- ments on the compressive properties of fly ash slurry under different pressures were conducted by using electro-hydrau- lic servo universal testing machine and pressure vessel for compression test designed by the authors. The results show that under the action of overburden stress,the volume of fly ash slurry is decreased,and the decrease rate of the slurry reduces gradually;the volume of the slurry is stabilized eventually. The equivalent coefficient of compressibility of fly ash decreases with the increase of pressure,and the relationship between them is found to be logarithmic. Based on the experimental results,the relationship between the weight of fly ash and the volume of compacted fly ash slurry is estab- lished under different grouting depths,and the formula of filling ratio is improved. The formula for the limit value of fly ash used for grouting is obtained,and the engineering verification is carried out,which provides a reasonable basis for the design of weight of fly ash in grout injection engineering.