The soft optic mode spectra of monoclinic I lanthanum pentaphosphates LnP5O14(Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb) are measured in the range of 20-300℃. The Ag-B2g and Ag-Ag soft modes in different LnP5O14 all soften to 20 cm-1 at ferroelastic transition temperature Tc. Another Bg-B3g soft mode softens to 38cm-1 at Tc. The coupling of Ag-B2gsoft mode with e5, plays an essential role in the phase trasition and the temperature dependence of soft-mode intensity can be described by means of the bilinear mode-coupling model. The increment of Tc from La to Tb in this system is owing to the lanthanum contraction.