Cadmium (Cd) is one of the most toxic trace elements. Cd accumulation by rice from paddy soils creates risks for human health. Compared with other common pollutants, Cd has a higher transfer coefficient. Rice improved by Si supplementation reduces Cd toxicity and decreases the accumulation of Cd in rice grains. The mechanism by which Si alleviates the effects of Cd remains unclear. The goal of this study was to investigate the effect of Si-rich substances on Cd uptake and translocation in rice plants grown in soils that were moderately and highly polluted with Cd. Special methods for determining soluble forms of Cd and Si in the apoplast and symplast of roots and shoots were used. The results showed that Cd translocation and accumulation in the soil-plant system differed at the medium and high levels of pollution. The application of Si-rich substances reduced Cd mobility in the soil via interaction between monosilicic acid and Cd ions and adsorption of Cd on the surface of Si substances. Cd accumulation in the rice roots and shoots was reduced by Si substances as well. At high Cd levels, Si application led to predominant Cd accumulation in the roots and the root apoplast, and reduced Cd translocation and accumulation in the shoots and the root symplast. The content of monosilicic acid in water extracted from fresh soil had a close relationship to the main parameters of mobility, transport, and accumulation of Cd in the soil-plant system. Several mechanisms could be responsible for Cd behavior in the soil-plant system. Firstly, the application of solid or liquid forms of Si reduced mobile and potentially mobile forms of Cd in the soil. Monosilicic acid can react with Cd and form insoluble silicates. Secondly, soil Cd can be adsorbed by solid Si-rich substances resulting in reduced Cd mobility. Additional plant Si nutrition increased the concentration of monosilicic acid in the apoplast and symplast of rice. As a result, most plant-absorbed Cd precipitated in the root apoplast and the translocation of Cd inside plant tissue was reduced. The Si-rich substances tested significantly affected the migration and accumulation of Cd in the soil-rice system. However, the degree of impact depends on the level of soil pollution and the ability of Si material applied to release monosilicic acid. Solid Si-rich substances applied can also adsorb Cd from the soil solution, and thus, reduce Cd mobility. Enhanced Cd uptake by rice lead to activation of plant defense mechanisms, and thus reduced Cd penetration through cell membranes to the symplast. In addition, some Cd could be associated with soluble organic molecules in the apoplast and symplast, which would increase the proportion of soluble forms in plant tissues. It was important to determine a soil Si parameter, which better reflected the effect of Si on Cd migration and accumulation in the plants. This parameter had the highest correlation with the uptake and accumulation of Cd and Si in rice plants and could be used for the prediction of the availability of Cd plant in the soil-plant system. However, these complex processes require further investigation.