A variable-diameter vacuum tank structure for a VD/VOD furnace comprises a vacuum tank which is of a variable diameter structure; the upper section of the vacuum tank is a large-opening-diameter straight barrel, the middle section of the vacuum tank is a conical barrel, the lower end of the vacuum tank is a small-opening-diameter straight barrel, and the large-opening-diameter straight barrel, theconical barrel and the small-opening-diameter straight barrel are connected in sequence; a flange and a sealing ring are installed at the top of the vacuum tank; the lower portion of the vacuum tankis provided with an air outlet; and an upper bracket is arranged on the small-opening-diameter straight barrel and provided with a slag receiving pan, a flow guide hole is formed in the lower end of the upper bracket, a lifting hook of a crane is put downwards into the vacuum tank and then directly hitches an open lifting hook in the center of the slag receiving pan, thus, steel slag is lifted outof the vacuum tank, and another new slag receiving pan can be lifted into the vacuum tank again. According to the variable-diameter vacuum tank structure, a straight barrel structure of the vacuum tank is changed into the variable diameter structure so that the slag removing efficiency, bleed-out treatment efficiency and maintenance safety of the vacuum tank can be improved.