The present invention relates to a method and an arrangement for pressure and temperature let down of autoclave discharge slurry, in particular in pressure oxidation or high pressure acid leach of metal containing ore. The method of the invention comprises a step of providing autoclave vent gas obtained from the autoclave to the top-entry flash vessel for inducing overpressure to the top-entry flash vessel and preventing boiling of the slurry during the transfer to next top-entry flash vessel. The invention further relates to an autoclave and pressure let-down arrangement adapted for providing autoclave vent gas to one or more top-entry flash vessels.