Abstract: The mechanical properties，morphologies of the fractured surfaces and the glass transition temperature were investigated for the epoxy resins cured with a mixed curing agent (aromatic diamine DETD and amine-terminated polyether D-400). The results show that with the addition of 40% relative mass fraction of D-400，the epoxy resin at room temperature exhibits the highest tensile strength of 82.52 MPa and Young‘s modulus of 2.30 GPa. And the tensile strength of the composites is enhanced by 6.3% while Young’s modulus by 14.4%. In addition，the cryogenic impact strength is enhanced by 14% at this content. SEM analysis of tensile fracture surfaces shows that the fractography tends to be irregular with the increase of flexible amine content，indicating that the fracture resistance is increased. Thermal analysis of the specimens shows that the glass transition temperature decreases with increasing the relative content of D-400. Finally，the mechanism of toughened epoxy resins at cryogenic temperature is discussed.