Abstract: The sP seismic phase of the Tonga-Fiji subduction zone have a clear image in the Gansu Digital Seismic Network and southeastern Gansu temporary observation array. In this paper, we extracted the weak precursors s₄₁₀P by slant stack method, and the s₄₁₀P precursors are produced by deep focus earthquakes that result from near-source underside reflection off "410". We compute the depth of "410" discontinuity by using the differential travel time between sP and its precursor s₄₁₀P near Tonga-Fiji subduction zone based on the modified velocity model (IASP91_Tonga) in this zone based on CRUST1.0 model. The depth of "410" discontinuity is about 398.5 km, and uplifted about 11.5 km, which maybe relate to the interaction of the subduction zone and the "410" discontinuity in the near-source zone. This result is of great significance to recognize the complex structure of subduction zones and objectively understand the dynamic process of the deep Earth.