The invention belongs to the field of rear earth metallurgy, and particularly relates to a pyrolysis method for preparing trivalent rare earth oxide. The method includes: heating a calcinatory to within a temperature range of 1000-1700 DEG C, mixing trivalent rare earth chloride solution with the concentration being 10-300g/L with compressed air with the flow velocity being 5-8000L/h, spraying the mixture into the calcinatory and calcinating for 1-40 minutes, and naturally cooling the calcinated calcination slag to obtain a powder product with the trivalent rare earth oxide content larger than 99wt%. Since the trivalent rare earth chloride solution is used as the raw material, no ammonium bicarbonate and toxicant oxalic acid is used during production, discharging of carbon dioxide during calcinating in the traditional process is avoided, and accordingly environment pollution is greatly reduced.