Abstract: Based on the Reynolds-Averaged Navier-Stokes equations with SST turbulent models, this paper studies the flow field around airship with forced envelop fluctuation, and concludes that the fluctuation of the envelop can cause significant change of the flow field. 1) When the upper and lower envelop are in the same phase, the decrease on wavelength can prick up the separation, and the time-averaged drag coefficient increases; 2) the drag coefficient increases with the increase of the fluctuation amplitude; 3) If the fluctuation amplitude is small, the wavelength has no effect no matter how small it is; 4) large frequency can induce serious unstability, and decrease drag coefficient; 5) the proper combination of wavelength and amplitude can decrease drag coefficient; 6) when the upper and lower envelop are in converse phase, the effects of wavelength and amplitude are similar to the case of same phase; 7) due to the offset of the upper and lower envelop, the time dependent drag coefficient may be negative temporally, but the time-averaged drag coefficient keeps positive.