Abstract: The effects of pulse frequency on TCGMAW(Twin-wire Co-pool Gas-shielded Metal Arc Welding) droplet transfer modes and weld appearances were investigated with highspeed photography and wavelet analysis.Under the welding parameters in this paper,the droplet transfer mode of the front wire was spray transfer and that of the back wire was projected transfer when the pulse frequency was high(140 Hz);the droplet transfer mode of both the front and back wires was projected transfer when the pulse frequency was low(40 Hz);the droplet transfer mode of both the front and back wires was spray transfer when the pulse frequency was moderate(60-100 Hz);the droplet transfer of both the front and back wires was one-droplet-per-pulse when the pulse frequency changed between 40 Hz and 140 Hz;and the weld appearances were preferable when the pulse frequency changed between 60Hz and 100 Hz.