To investigate the uniaxial tensile mechanical properties of high-early-strength high ductility concrete (HES-HDC) at different PE fiber diameters and volume fractions, 17 groups of thin-plate specimens were designed and loaded under uniaxial tensile tests. The effects of PE fiber diameters (22 μm and 25 μm) and fiber volume fractions (1.00%, 1.25%, and 1.50%) on stress-strain curve, tensile strength, and strain of HES-HDC specimens at different curing ages (2 h, 24 h, 7 d, 28 d and 56 d) were investigated. A tensile toughness evaluation method suitable for the characteristics of HDC was proposed based on the test results. The results show that the HES-HDC stress-strain curve exhibits strain-hardening characteristics under uniaxial tensile loading, and the failure process is carried out with multiple cracks. The tensile strength and strain of HES-HDC at 2 h reach over 3.29 MPa and 0.88%, respectively, and the tensile strain at 28 d could remain more than 1.28%. When the fiber volume fraction is 1.00%, small-diameter fiber is beneficial to the tensile strain of specimens, while large-diameter fiber is beneficial to the tensile strength of specimens. Considering the tensile strength and strain of specimens with large-diameter fiber, the optimal fiber volume fraction is 1.25%. The proposed tensile toughness evaluation method can evaluate the tensile toughness of HDC during its whole loading process. In addition, the tensile toughness index of HES-HDC decreased with the increase of curing age, while the tensile strength coefficient increased. The tensile toughness of specimens with small-diameter fiber is higher than that of specimens with large-diameter fiber. The tensile toughness of specimens is the highest when the fiber volume fraction is 1.25%.