A new visualization method for studying the damage to gel structure caused by high salinity ions is explored by using the characteristics of suppression image signal of Mn ²⁺ and nuclear magnetic resonance (NMR) imaging technique. The diffusion and distribution characteristics of Mn ²⁺ in porous media-gel system were studied based on manganese chloride static diffusion and gel flooding experiments, and the gel’s nuclear magnetic image and displacement pressure were tested. The results show that the diffusion of Mn ²⁺ conforms to the Fick diffusion law in porous media-gel system, and the diffusion speed of Mn ²⁺ increases and the area of gel image decreases gradually with the increase of concentration, and the image of gel decreases faster and the pressure drop of water drive is larger in flooding experiment of manganese chloride with higher concentration. Reaction-diffusion model with the reaction of Mn ²⁺ with gel was established to study the concentration distribution characteristics of Mn ²⁺. The model is validated by comparing the results with magnetic resonance imaging (MRI) experiments and the diffusion coefficient of Mn ²⁺ equals 1.6 mm ²/h, and the minimum concentration of Mn ²⁺ to impact gel NMR image signals is 2.5 g/L. The above results show that the diffusion of Mn ²⁺ into the gel in the rock core inhibits the imaging signal of the gel and damages its strength, and the greater the concentration is, the greater the influence. Increase of adsorption amount of gel and reaction rate, reduction of diffusion time, and addition of ion adsorption isolator all can reduce the impact of Mn ²⁺ on the gel.