Excellent thermal neutron absorption performance of boron expands the potential use of boron rich slag to prepare epoxy resin matrix nuclear shielding composites. However, shielding attenuation behaviors and mechanism of the composites against gamma rays are unclear. Based on the radiation protection theory, Phy-X/PSD, XCOM, and 60Co gamma ray source were integrated to obtain the shielding parameters of boron rich slag/epoxy resin composites at 0.015–15 MeV, which include mass attenuation coefficient (μt), linear attenuation coefficient (μ), half value thickness layer (HVL), electron density (Neff), effective atomic number (Zeff), exposure buildup factor (EBF) and exposure absorption buildup factor (EABF).μt, μ, HVL, Neff, Zeff, EBF and EABF are 0.02–7 cm2/g, 0.04–17 cm−1, 0.045–20 cm, 5–14, 3 × 1023–8 × 1023 electron/g, 0–2000, and 0–3500. Shielding performance is BS4, BS3, BS3, BS1 in descending order, but worse than ordinary concrete. μ and HVL of BS1-BS4 for 60Co gamma ray is 0.095–0.110 cm−1 and 6.3–7.2 cm. Shielding mechanism is main interactions for attenuation gamma ray by BS1-BS4 are elements with higher content or higher atomic number via Photoelectric Absorption at low energy range, and elements with higher content via Compton Scattering and Pair Production in Nuclear Field at middle and higher energy range.