Abstract: By using Fenlon's theory the effect of degenerate parametric amplification occurred due to the nonlinear interaction of a weak sound signal with frequency f₁ and a powerful pump wave with frequency f₂=2f₂ has been studied in this paper.The theory shows that this effect of amplification depends obviously on the initial phase of the signal and the maximum amplification of weak sound can be obtained at initial phase of weak sound φ₁=π/2 or at initial phase of pump wave φ₂=π.The experiment was carried out in a long progressive wave tube with 13 m length and 5 cm diameter.In experiment,the dependence of amplification of weak sound on its initial phase at different distance and the relationship between amplification of weak sound and distance at certain initial phase correspond to maximum and minimum amplification have been studied.Experiment shows that maximum amplification of weak sound,occurred at φ₁=π/2 or φ₂=π,is 1.8-2 dB.The experimental results coincide quite well with the theoretical prediction.