In view of the deep stirring treatment of soft foundation mucky soil in a certain waterway engineering，we study the effects of geopolymer with fly ash（FA）and slag（S）on the properties of mucky soil. Geopolymer and conventional amendments of 10%，20%，and 30% are added into field mucky soil，and specimens of different moisture contents are prepared. Unconfined compressive strength（UCS）tests，flexural-tensile modulus tests，and scanning electron microscope（SEM）imaging tests are carried out to evaluate the characteristics of mechanical property and microstructure changes of the improved soil and thereby obtain a reasonable amendment amount. The following observations can be drawn from the results：1）Geopolymer improvement with FA+S and alkali activation solution contributes to a more compact soil microstructure and significantly improves the strength and stiffness of the mucky soil. 2）Considering the economic cost，the desirable amount of geopolymer amendment is 20%（5%FA+15%S），and its applicable moisture content range is wide，specifically 0.75 WL-1.25 WL.