The relationship between average velocity of phase and/or domain interface V and the effective phase transformation (PT) driving force in the process of diffusionless (DL) PT △G＇ (the difference between PT driving force AG and the resistence △GR, △G＇=△G - △GR) can be expressed as V = φ(△G - △GR). Consider a monodirectional varying external field (intensity is ξ and varying rate is ξ) exerted on a DLPT system, the phase interface moves and then the DLPT takes place when △G provided by the external field increases to a critical value △GR. If a hamonic external stress σ = σ0 sin ωt which can interact with the moving interface exerts on the system and the coupling factor is n,an expression between dynamic relation of interface V = φ(△G - △GR) and the internal friction in the DLPT process Q-1, related modulus defect △M/M, PT rate dF/dξ and the PT strain ε0 can be derived as d lnφ(△G＇)/d△G＇= Q-1ω/n2M(dF/dξ) = (△M/M)ω/nMε0(dF/dξ)ξ, or and (△M/M)Q-1=ε0/n. where ω is frequency of hamonic stress, M is modulus related to vibration mode. A gerenal solution of the interface dynamic equation can be obtained as V = ∑(±n)/(α≠-1) Aα exp[((△G - △GR)/△Gα*)α+1/(α+ 1)] +∑(m)/(β0) Aβ(△G-△GR)/△Gβ*)β where Aα,Aβare coefficient, △Gα* and △Gβ* are resistance parameters related to the high velocity interface. The specific solution and the value of △GR can be determined by the experimental data of the internal friction in the DLPT with diffrent ξ. The equation of (△M/M )/Q-1 can be used to determine whether the soft mode contributes in the process of DLPT.