Abstract: In the two-step vitrification process, the high-level liquid waste can be reduce acidity by chemical denitration, and the commonly used chemical denitration agents are formic acid, formaldehyde, sucrose, etc. In this paper, the denitration of simulated high-level liquid waste and the precipitation behavior in the denitration process was studied with formaldehyde as the chemical denitration agent. The simulated high-level liquid waste was denitrated at 90 ℃ with denitration ratio in the range of 1.0 to 2.0. After denitration, each substance was analyzed by ICP-OES, XRF, SEM-EDS, Raman, XRD and TG. The results show that the content of nitrate in the waste stream after denitration is significantly reduced, and the content of nitrate gradually decreases and the content of formaldehyde increases with the increase of denitration ratio. The precipitation consisting of Zr, Mo, La, Ce, Nd, Fe, Te, Pr, Cs, Sm, Cr, Sr, Y, Co, Ni appeares in the denitration process. The precipitation is formed by two processes, one process is the formation of granular crystals Ln₂Zr₃(MoO₄)₉ (Ln=La, Ce, Nd, Pr, Eu, Sm) and MoO₂; the other process is the formation of an amorphous powder substance consisting of O, Fe, Zr, Mo, and Te. The thermal decomposition of denitration products mainly occurs below about 360 ℃.