Polyethylene/carbon nanotube (PE/CNT) composites with high hydrogen content as a kind of structural material for space radiation shielding have extensive potential applications in future aerospace field due to their unusual radiation shielding, lightweight, and easy processing. In the space irradiation environment, the composites are sensitive to radiation damage, which changes their microstructures, directly affecting their mechanical performances and shielding effectiveness. Low energy electrons (200 keV) are important radiation environmental factors. Effects and mechanisms for mechanical damage of PE/CNTs composites induced by low energy electrons are studied, which has important academic value and practical significance. Previous research mainly involves the qualitative evaluations of the changes in the mechanical performance index of polymer nanocomposites. The inner relationship between microstructural change induced by radiation and mechanical behavior of the nanocomposites, especially in the PE/CNTs composites has not been studied in depth so far. In this paper, low-density polyethylene (LDPE)/ multi-walled carbon nanotube (MWCNT) composites are chosen as a research object. Based on 110 keV electron irradiation, tensile deformation mechanism of the LDPE/MWCNT composite is studied. The synchrotron radiation X-ray small angle scattering (SAXS) and wide angle diffraction (WAXD) are used to reveal the real-time microstructure evolutions of the nanocomposites after and before irradiation in the process of stretching. Tensile deformation mechanisms of LDPE/MWCNT composite after and before the 110 keV electron irradiation are revealed. Experimental results show that the tensile deformation behavior for the irradiated LDPE by 110 keV electrons is different from that for unirradiated sample. The electron irradiation increases the tensile strength of the LDPE/MWCNT composite and reduces the elongation at break. Moreover, with increasing the irradiation fluence, the tensile strength and the elongation at break of the LDPE/MWCNT composite significantly increases and decreases, respectively. The electron irradiation could hinder the deformations of the LDPE matrix including crystalline case and amorphous case, constrain the fragmentation of original lamellae, the directional arrangement of the MWCNTs, the formation of new crystal and the rotation of lamellae, especially in higher irradiation fluence. During tensile deformation, the main strengthening mechanism for the irradiated LDPE/MWCNT composites by the 110 keV electrons is crosslinking strengthening effect in LDPE matrix. On the other hand, enhanced interaction (mainly interface strengthening) between MWCNTs and LDPE matrix induced by irradiation is attributed to the main strengthening mechanism for the irradiated LDPE/MWCNT composites. These results could provide a theoretical basis and technical support for the reasonable design and successful application of CNT-based polymer composites as structural material for space radiation shielding.