Decitabine is a cytosine analogue and is a specific DNA methyltransferase (DNMT) inhibitor. It directly inhibits DNMT by phosphorylating DNA and inhibits DNMT, thereby reversing DNA methylation and inducing tumor cells to Normal cell differentiation or induction of tumor cell apoptosis.Diffuse large B-cell lymphoma (DLBCL) is the most common pathological type in non-Hodgkin's lymphoma. The first-line chemotherapy regimen using Rituximab+Cyclophosphamide+Doxorubicin +Vincristine+Bonisone（R-CHOP）significantly increases the remission rate and disease-free survival of patients with DLBCL, but it is difficult to partially relapse. Long-term remission and survival rates in treating patients are not satisfactory.Due to the greater cardiac toxicity of adriamycin, more patients can not be uncomfortable, so the COP program is also widely used in patients with DLBCL, and achieved a good response rate.In 2008, the FDA had approved decitabine for the demethylation treatment of Myelodysplastic syndrome（MDS）. Over the years, good initial remission rates and better long-term survival rates have been achieved in patients with MDS.There are also a variety of clinical trials of decitabine for patients with solid tumors that have achieved significant clinical efficacy.Due to the high side effects of high-dose decitabine, patient tolerance is poor. Therefore, the purpose of this study was to evaluate the efficacy and safety of low-dose decitabine combined with Cyclophosphamide+Vindesine+Bonisone(COP) regimen (D-COP) 4-6 course of treatment for relapsed and refractory diffuse large B-cell lymphoma.