The highly cross-linked epoxy cured network suffers from the brittle nature and poor elongation, which hinders many potential applications. In this study, a biology-based cardanol NC-514S-modified polyether amine D230 was synthesized and used as an epoxy hardener (NC-D230). The effects of NC-514S content on the mechanical and thermal properties of the prepared samples were investigated. At the same time, we used cyclic stretching to further characterize its mechanical strength, and characterized the curing agent before and after modification through SEM and FTIR. Mechanical tests on the samples showed that the increase of NC-514S content results in lower tensile strength and Young’, s modulus but an increase in elongation-at-breaks. DSC and TGA analyses showed a decreasing trend of glass transition temperature with the addition of NC-514S. Typically, the epoxy thermoset without/with 75% (by wt) NC-514S-modified D230 decreased tensile strength from 61. 7 to 17. 74 MPa, and Young’, s modulus from 496 to 11. 56 MPa. It increased elongation from 13. 43% to 97. 97%, and decreased glass temperature from 87. 5 to 51. 9 °, C by DSC and 87. 8 to 54. 7 °, C by DMA, respectively. This novel bio-based renewable cashew phenol-modified polyetheramine may provide an ideal candidate for toughening an epoxy-based thermosetting materials. At the same time, it also provides a new way for the toughening of epoxy resin.