Supersulfated cement (SSC) is considered as an eco-friendly alternative to ordinary Portland cement. However, the investigations of SSC concrete with fly ash (FA) and granulated blast furnace slag (GBFS) are still limited. This paper presented an investigation on effects of compositions on cracking potential and viscoelastic behaviors of SSC concrete at early age. FA and GBFS were introduced to replace SSC. The replacement ratio was 0%, 30%, and 50% by weight of SSC. Mechanical properties and free shrinkage were determined. Viscoelastic behaviors including stress relaxation and creep were quantified by restrained ring test with different degrees of restraint. Results demonstrated that the mechanical properties of SSC concrete were significantly reduced by replacing SSC with FA and/or GBFS. At a same replacement ratio, combined addition of FA and GBFS enhanced mechanical properties. Shrinkages were mitigated by replacement of FA and/or GBFS and decreased with an increase on FA replacement ratio. A high replacement ratio induced a rapid increase on cracking potential in the first few days. The replacement of FA and/or GBFS decreased the relaxed stress of SSC concrete. The available results of this paper might help to better understand the application potential of SSC concrete in engineering practices.