To improve the development effect of routine steam stimulation in the middle and late period，the swelling and viscosity reduction effect of air and CO₂ on common heavy oil was probed using PVT. Air assisted steam stimulation and CO₂ assisted steam stimulation was developed by laboratory experiments，and the development effects of two kinds of stimulation was evaluated through index analysis of return water rate，injection production ratio，average temperature of reservoir and recovery degree. Results showed that，after air and CO₂ dissolved in crude oil，the viscosity decreased and the volume factor increased，however，the effect of injecting CO₂ was better. The sweep efficiency of steam and average temperature of reservoir increased with the increase of stimulation cycle. The sweep efficiency and average temperature of the gas assisted steam stimulation was higher than that of the routine steam stimulation in the same cycle. The development effect of adding air and CO₂ was obvious in the middle and late period of routine steam stimulation，which increased the return water rate，oil steam ratio and injection production ratio. Compared with the routine steam stimulation，the air assisted steam stimulation and CO₂ assisted steam stimulation could greatly improve the recovery degree，and the development effect of CO₂ assisted steam stimulation was the best.