By means of trans-dermal permeation on rat skin in vitro, the effect of low-frequency ultrasound (40 kHz) on trans-dermal permeation of (Protein transduction domain-superoxide dismutase, PTD-SOD) has been investigated. The results show that the enhancement ratios of ultrasound and Azone are 3.40 and 2.52, respectively, when the trans-dermal permeation time is 8 h. In the presence of low-frequency ultrasound, when the initial pH value of drug solution is 6.0, the activity of PTD-SOD in receiver cell reaches 1598.00 U, the maximum value. Under the action of low-frequency ultrasound, the trans-dermal permeation of drug varies as a function of the initial PTD-SOD concentration according with a Fick-type scattering law, and the coefficient of permeation is 0.011. These results demonstrate that Low- frequency ultrasound can significantly improve trans-dermal transport of PTD-SOD, and the effect is superior to Azone. Both of the initial pH value of drug solution and the initial PTD-SOD concentration can affect the permeation rate of the PTD-SOD. And the best initial pH value and drug concentration is 6.0 and 15.00kU·mL,-1, respectively.