Abstract: The effects of CaO addition on structure and arsenic immobilization in xCaO-(100-x)(30Na₂O-2.8B₂O₃-25.2Fe₂O₃-42P₂O₅)(x=0, 5, 10, 20 mol%)(sodium-iron-boron-phosphorous, SIBP) glasses are studied and the results indicate that the addition of 20 mol% CaO in the SIBP glasses decreases the leaching concentration of arsenic from 3.14 mg/L to 0.99 mg/L, and the ratio of arsenic immobilized in the glass increases from 60.78 % to 88.87 %. The Q2 bridge oxygen structure can be broken by the addition of CaO, which leads to depolymerization of SIBP glasses network. Simultaneously, CaO added in the glasses can depolymerize the phosphate chains and participate in the network by forming chemical bonds such as Ca-O-P, Ca-O-As bonds in the glasses which will strengthen the glass structure and compact the glass density. Moreover, with the increase of CaO, the SIBP glasses transition temperature (T_g) and △T exhibit tendency of increasing, which demonstrates that the SIBP glasses structure has been strengthened. Besides, the cross-linking between non-bridging oxygen of phosphate and Ca²⁺ also has great impacts on the arsenic immobilization in the SIBP glasses.