The work aims to study the mechanical properties of aged unidirectional UHMWPE sheets, and learn the aging resistance of body armors in natural environment and provide scientific guidance for the use and storage of bullet-resisting equipment. The quasi-static tensile mechanical properties of UHMWPE were tested by 3D-DIC and a universal material testing machine. The experimental results showed that the stress-strain curve of unidirectional UHMWPE sheets after ten years of natural aging was still approximately linear, but the average tensile modulus was only 18.0 GPa. The deformation characteristics of unidirectional UHMWPE sheets specimens were analyzed based on the whole field strain variation process in the test process. It was found that the strain distribution in the analysis area was uniform during the tensile process. After ten years of natural aging, the average tensile modulus of unidirectional UHMWPE sheet soft body armor is obviously reduced, and the bullet-resisting performance cannot meet the requirements of use.