Abstract: A typical simulated high level radioactive sludge is prepared according to the analysis data of high level liquid waste (HLLW) and sludge. The sludge is got after a concentration and denitration process of the HLLW by formic acid. The physical and chemical properties of the sludge are studied. It mainly consists of BaSO 4、K 3(Ba) PO 4· 6MoO 3 and Na(Fe)NO 3 crystals. The mechanism of the sludge formation is discussed.