The invention discloses a process for producing vanadic anhydride by vanadium slag. The steps of the process comprise as follows: a, adding alkali metal salt in the vanadium slag, and mixing with each other uniformly, a mixture is obtained. b, laying the mixture obtained by step a into a calcining furnace to oxidize and calcine, vanadium slag grog is obtained. c, the calcined vanadium slag is discharged out of the furnace, fast cooling and performing water immersion. d, filtering the mixture obtained by step c, filter liquor is obtained. e, removing impurities in the filter liquor. f, adjusting the pH value of the filter liquor obtained by step e, adding ammonium salt to precipitate vanadium and filtering the filter liquor, then, ammonium peroxovanadate filter cake and ammonium metavanadate filter cake are obtained. g, calcining the filter cake after vanadium precipitation obtained by step f, and vanadic anhydride is obtained, wherein the content of the vanadic anhydride in the vanadium slag used in step a is 2.0%-8.0%. The invention has the advantages of simple and easy, low facility request, convenient operation and low cost, further, the residual quantity of the vanadic anhydride in waste slag is 0.55%-1.0%, which has fine economic benefit and social benefit.