Abstract: The phase formation of Si₃(PO₄)₄ for P₂O₅/SiO₂ catalysts has been investigated by XRD, reaction chroma tography and amine adsorption.The results showed that the 400~500℃ is important range of phase formation, if the calcination temperature below them, with the increase of temperature, the crystallinity and crystallite size will be advance, if the temperature higher than this range, such as 500~800℃, the crystallinity and crystallite size retain constants.As it happens, it can be found from the variations of DMDO delivery volume and surface acid concentration, there are indicated turning points in 400~500℃.But also the relative surface area of crystal phase of Si₃(PO₄)₄ and relative yield of isoprcne reach maxmarn within the range of temperatures 500~800℃.The reasons for the adhesion between catalyst particles arc also explained.