Abstract: Two kinds of ultrahigh molecular weight polyethylene (UHMWPE) fiber, pristine and in-situ modified by carbon nanoparticles (CNPs), were used to prepare cross-ply unidirectional UHMWPE fiber/waterborne polyurethane (WPU) composite laminates by special process (winding, compound, and hot-pressing). The UHMWPE fiber/WPU composite laminates were impacted by mild-steel core projectile with size of 7.62 mm×39 mm at impacting velocity of (720±10) m/s. The ballistic behavior of UHMWPE fiber/WPU composite laminate was investigated by the X-ray computed tomography (CT) technology. The results show that the CNPs in-situ modification of CNPs-UHMWPE fiber may improve the single anti-penetration of its laminate, and this method may diminish the ability of resisting multiple strikes of its laminate. In addition, the penetration process of UHMWPE fiber/WPU composite laminate will be divided into three stages in a non-penetrating event: Shear plug, fracture failure and plastic deformation of the remaining sub-laminate. Moreover, the thickness ratios of every stages are about 11.51%, 44.40% and 44.09%, respectively. The delamination behavior of post-impact laminate is mainly distributed in the second stage, and approaches the impact point. Furthermore, the damage area of each projectile to the laminate may be contained in a circle with the impact point as the center and the dimeter of 70 mm.