Carotenoids are antioxidant compounds have been used for human benefits and in many industrial applications. The halophilic microalga Dunaliella parva is rich with natural carotenoids such as β-carotene. To enhance carotenogenesis, an external phytoene synthase gene (PSY) from Synechocystis sp. strain PCC 6803 was successfully cloned into D. parva cells (D-PSY). D-PSY showed higher carotenoids value (3.8 mg g−1 DW) compared to the wild-type (WT) strain (2.5 mg g−1 DW). For further assessment, both WT and D-PSY strains were subjected to two different stresses (polyethylene glycol (PEG) and CaCl2). Application of CaCl2 enhanced the chlorophylls (Chl a, Chl b) and carotenoids contents of both strains compared to PEG. The maximum carotenoids contents were obtained by treatment of D-PSY strain with 30 ppm PEG (7.0 mg g−1 DW) and 60 ppm CaCl2 (6.0 mg g−1 DW). The soluble carbohydrates contents of D-PSY cells recorded high value (83.5 mg g−1 DW) compared to that of WT strain (52 mg g−1 DW). While 60 ppm CaCl2 significantly stimulated the carbohydrate contents of both WT and D-PSY (58.4 and 117 mg g−1 DW, respectively), 30 ppm PEG showed an inhibitory effect (50.4 and 58.5 mg g−1 DW of WT and D-PSY, respectively). The antioxidant activities of the carotenoids samples were analyzed through five different assays. Generally, the antioxidant activities of D-PSY carotenoids were superior to that of WT. Moreover, stress applications (30 ppm PEG and 60 ppm CaCl2) enhanced the antioxidant activities of the carotenoids samples of both strains.