During the mining and metallurgy of non-ferrous heavy metals, a large number of arsenic-containing compounds are exposed to the environment, posing great environmental risks to the surrounding water and soil. Due to their good arsenate affinity, iron ions and their compounds are the main components of commonly used chemical arsenic fixation agents. Whether in arsenic pollutant solidification or arsenic-contaminated site remediation, silicate and hydration processes are important physical barriers to arsenic. Based on this, we synthesized an iron-based silicate gel and evaluated its performance for the solidification/stabilization of typical arsenic compounds [Na3AsO4, Ca3(AsO4)2, AlAsO4, and FeAsO4·2H2O] and arsenic-containing sludge from non-ferrous metallurgy, and explored the arsenic fixation mechanism. The results showed that the iron-based silicate gel with a Fe/Si molar ratio of 1:4 could effectively immobilize the arsenic-containing compounds (Na3AsO4 and FeAsO4·2H2O). However, in the process of curing Ca3(AsO4)2 and AlAsO4, due to the competitive reaction between arsenate and silicate, the toxic leaching of arsenic was higher than that in the process of uncaring. The introduction of CaO could inhibit the competitive reaction, improve the arsenic fixation rate of Ca3(AsO4)2 and AlAsO4, and reach more than 98% of the arsenic fixation efficiency. The synergistic effect of Fe and Ca co-precipitation and physical immobilization is responsible for the immobilization/stabilization of arsenic-containing compounds. The core-shell structure with arsenic-containing compounds as the core and iron-based silicate gel/C-S-H gel as the shell separated arsenic species and reduced toxic leaching when in contact with the surrounding environment. The long-term stability showed that the iron-based silicate gel-cured arsenic-containing waste remained highly stable at pH=8 for 30 days. The CaO-assisted iron-based silicate gel proposed in this work showed great potential for the immobilization of arsenic-containing wastes and arsenic-contaminated land and provided a new way to solidify arsenic-containing pollutants.