Steel slag powder and fly ash solid waste materials were used to replace part of cement, and lime and NaOH were used as activators to form a new composite curing agent to modify mucky soil. The mass change rate, moisture content and unconfined compressive strength of solidified soil samples modified by composite curing agent under seawater erosion were studied. SEM was used to analyze the microstructure changes of soil particles, the distribution of cements and their interaction with soil particles in solidified soil under seawater erosion. The results show that the mass change rate of 28 d solidified soil is generally lower than 7 d solidified soil, which indicates that the longer the curing age of solidified soil is, the more stable the internal structure is. The moisture content of each sample decreases with the increase of seawater erosion concentration. When the content of composite curing agent is 20% (SLFA-CS-20%), the moisture content of solidified soil sample is the lowest. With the increase of the concentration of erosion ions in seawater, the strength of solidified soil gradually increases with the increase of erosion time. Mucky soil can produce a large number of cementitious materials and calcium vanadium stone under the action of composite curing agent, so as to improve the unconfined compressive strength of composite solidified soil.