This paper aimed to study the early-age properties and viscoelastic behaviors of supersulfated cement (SSC) concrete with different replacement ratios (15%, 30%, and 50%) of fly ash (FA). Ordinary Portland cement (OPC) concrete was also designed for reference. Mechanical properties and free shrinkage were tested. Stress relaxation and creep as the significant viscoelastic behaviors were investigated in restrained ring test with different degrees of restraint. The results showed that later mechanical properties of SSC concrete approached or exceeded that of OPC concrete while replacement of FA caused a significant reduction. The workability of SSC concrete was improved with FA replacement. Free shrinkage was mitigated by FA due to the significant reduction on autogenous shrinkage of SSC concrete. At a high degree of restraint, SSC concrete with a FA replacement ratio of 50% exhibited a high cracking resistance. At a relatively low degree of restraint, cracking potential was reduced by FA replacement due to its significant mitigation on shrinkages. Relaxed stress of SSC concrete decreased with an increase on FA replacement ratio. The nonlinear relationship between relaxation and creep coefficients of SSC concrete was established regardless of FA replacement ratios and degrees of restraint, which was different from OPC concrete. The results obtained in this paper might promote the reliable applications of SSC concrete in the future.