ObjectiveTo analyze the causes of death of patients with asthma-chronic obstructive pulmonary disease overlap syndrome (ACOS). MethodsA total of 493 patients admitted between January 2006 and Octomber 2015 were respectively analyzed, including 348 asthma patients and 145 ACOS patients. The patients was divided into a survival group and a death group based on the outcome. The ACOS patients were divided into three subgroups based on FEV1% pred level (≥80%, 50%-80%, and < 50%, respectively). The basic characteristics and causes of death were analyzed using χ2-test, t-test and Fish-test based on data type. ResultsThe age (t=3.457, P < 0.001), male proportion (χ2=15.394, P < 0.001) and smoking history (χ2=12.418, P=0.002) had significant differences between the survival group and the death group. The proportion of ACOS patients was higher in the death group (42% vs. 27%, χ2=7.033, P=0.008), and the mortality was also higher in the ACOS patients (21% vs. 12%). The proportion of male patients was higher in the ACOS patients than that in the asthma patients (86% vs. 38%, P < 0.001). The leading three causes of death in the ACOS patients were malignant diseases (45%), pneumonia (26%), and cardiovascular diseases (16%). Malignant diseases were the main cause of death in the ACOS patients with FEV1% pred≥50%, while pneumonia was the main cause of death in those with FEV1% pred≥50%. There was no significant difference in cause of death distribution between three subgroups with different FEV1% pred (P=0.318). ConclusionThe main cause of death of ACOS patients is malignant diseases, the followed are pneumonia and cardiovascular diseases.