Abstract: The coagulation mechanism is abnormal in malignant tumor patients， such as the increased platelet count and increased platelet aggregation. Therefore， the cancer associated thrombosis is one of the most common complication in malignant tumor patients. According to the epidemic investigation of maglignant tumor and thrombus， the risk of venous thromboembolism（VTE） is about 4-7 times higher in cancer patients than that of nontumor patients. Risk factors for cancer associated thrombosis include tumor types， patients characteristics and treatment related factors. We can prevent and treat tumor associated thrombosis according to its biological markers and risk models. The heparin or low molecular weight heparin is often applied in the treatment of cancer patients with VTE. But the thrombocytopenia often occurs after the treatment of heparin therapy， which causes the skin and organ extensive bleeding in cancer patients. Therefore， this paper will briefly review the pathogenesis of maglignat tumor associated thrombosis， the clinical and laboratory diagnosis of thrombocytopenia after the heparin treatment and the progress of clinical treatment， which can be useful for discussion and reference for the clinicians.