Abstract: The blocking reason of wastewater reinjection well was studied by analyzing components of sediment on the bottom of reinjection tank and supernatant water, which provides practical and theoretical basis for optimizing. Results of these experiments show that grain size of sediment is large; oil content of sediment is high; 50% or more of sediment qualities are CaCO₃ and acid non-soluble substance; bacteria content in supernatant water exceeds the national standard obviously. The conclusion is: wastewater reinjection wells may be blocked because of suspended solids or pore channels blocked by oil residue; reinjection wastewater and formation water is not compatible; self-induced scaling congestion and microbial reproducing organisms congestion.