Plastics, combined with plasticizers, have been widely utilized worldwide. Microplastic fibers (MPFs) and dibutyl phthalate (DBP) account for the most predominant microplastics and plasticizers detected in freshwater ecosystem, with their joint toxicity being limited studied. In this study, we employed freshwater algae (Chlorella vulgaris) as toxicity test model organism to assess their growth, photosynthesis, metabolism, and oxidative response when exposing to different concentrations of polypropylene MPFs and the co-exposure of DBP. In addition, the toxic interaction between MPFs and DBP was assessed by combining the integrated toxicity value (Integrated Biomarker Response version 2, IBRv2) and the mixture toxicity index (Effect Addition Index, EAI). Our results demonstrated significant toxic effects of MPFs and DBP on C. vulgaris, and highlighted their dynamic interactions with C. vulgaris. Specifically, when combining with DBP, MPFs with high concentrations exhibited significantly increase in algae growth inhibition, photosynthetic pigment contents (Chl-a, Chl-b, and carotenoids), protein contents, and oxidative enzymes (SOD, CAT, and MDA). In terms of integrated toxicity values, higher IBRv2 values were recorded by the combined exposure of MPFs and DBP in contrast with the sole exposure groups, indicating that the combined exposure caused more severe damage to photosynthesis, oxidation and metabolism. In addition, our study recorded synergistic combined toxicity when MPFs were in high concentrations, whereas antagonistic combined toxicity when MPFs were in low concentrations. Our study highlights the MPFs concentration-dependent combined toxicity (synergistic or antagonistic) when exposing to microplastics and plasticizers in freshwater ecosystems.