The invention discloses a heat treatment method after forging of ultra-supercritical high and medium pressure rotor steel. The heat treatment after forging of ultra-supercritical high and medium pressure unit rotor steel X12CrMoWVNbN10-1-1 and Cr10.5Mo1W1NiVNbN is carried out by the steps of: step 1. austenite homogenization treatment; step 2. carrying out structural transformation when balanced structure transformation temperature is reached by air cooling; step 3. overall structure softening; and step 4. cooling in a speed limit furnace till discharging. Since the rotor steel is directly put into the furnace for austenization after being forged, the invention has the advantages of short time and good austenite effect, not only eliminates the forging stress, but realizes the homogenization of inner and outer structures of forging. The heat treatment method after forging has the advantages of simple operation, low cost and low energy consumption, and can meet the property requirement of workpieces.