Objective To evaluate the effects of different inspiratory flow waveforms on the respiratory function of patients with severe exacerbation of asthma during mechanical ventilation. Methods Twenty-one patients with severe exacerbation of asthma were ventilated with square waveform and decelerating waveform respectively for 30 minutes when the tidal volume was set at 6 mL/ kg, 8 mL/ kg and 10 mL/ kg in ICUof Zhejiang Hospital of Integrated Traditional Chinese and Western Medicine fromJanuary 2006 to December 2007. Meanwhile shunt fraction ( Q·S /Q·T ) , dead space value ( VD/VT ) , airway peak pressure ( Ppeak ) , plateau pressure ( Pplat) ,intrinsic positive end-expiratory pressure( PEEPi) and arterial blood gas analysis were measured. Results The Q ·S /Q·T in the decelerating waveformgroup was less than that of the square waveform group( P lt;0. 05) when tidal volume was set at 6 mL/ kg. When tidal volume was set at 10 mL/ kg, PEEPi and VD /VT in the decelerating waveform group were higher than those of the square waveform group. On the contrary, the Ppeak was lower than that of square waveform group( P lt; 0. 05) . Conclusion For patients with severe exacerbation of asthma treated with mechanical ventilation, decelerating waveform is preferable at low tidal volume( 6 mL/ kg) , and square waveform is preferable at high tidal volume( 10 mL/kg) .