We study the discontinuities beneath Tonga subduction zone using the deep earthquakes recorded by the Pacific Northwest Seismic Network (PNSN). The multiple discontinuities around 660 km beneath Tonga subduction zone are found using converted phase SdP. The amplitude ratios of the converted phase with the direct P wave (Ac/Ap) are also determined. The largest number of robust converted phases with large Ac/Ap comes from the depth range of 660--690 km, its peak is at 680 km; the second one appears in the depth range of 700--745 km, its peak is at 740 km. The two discontinuities are obviously depressed by the subduction slab. According to experimental and numerical studies, a possible explanation of the observation is that the first discontinuity is formed by the transition of γ-spinel to perovskite and magnesiow¿stite and the second is formed by the transition of ilmenite to perovskite.