Abstract: In recent years,the morbidity and mortality of patients with malignant tumor has been increasing continually.The platelets significantly increased were in patient with malignant tumor in vivo.The platelets play an important role in the pathogenesis of malignant tumors.We should make a distinction between primary thrombocythemia and secondary cases thrombocytosis,because there are more difference in their treatment and prognosis.There was close relationship between thrombocythemia and patients with malignant tumor.The platelet could be excitated and activated by the malignant tumor cell.The platelet could promote tumor cell proliferation,metastasis and platelet tumour cell embolization.Blood platelet count be known as prognosis independence factor of patient with malignant tumor.We should control blood platelet count.It is possible that platelet suppressant drug treat malignant tumor.We briefly sum up cause of thrombocythemia and relationship with malignant tumor and so on.