To examine patterns of human immunodeficiency virus (HIV) cause-specific deaths, risk factors, and the effect of interactions on mortality, we conducted a retrospective cohort study in Zhejiang, China, from 2006 to 2013. All data were downloaded from the acquired immune deficiency syndrome (AIDS) Prevention and Control Information System. The Cox proportional hazards model was used to assess predictors of cause-specific death. The relative excess risk due to interaction and ratio of hazard ratios (RHR) were calculated for correlations between HAART, late diagnosis, and age. A total of 13,812 HIV/AIDS patients were enrolled with 31,553 person-years (PY) of follow-up. The leading causes of death of HIV patients were accidental death and suicide (21.5%), and the leading cause of death for those with AIDS was AIDS-defining disease (76.4%). Both additive and multiplicative scale correlations were found between receiving HAART and late diagnosis, with RERI of 5.624 (95% CI: 1.766-9.482) and RHR of 2.024 (95% CI: 1.167-2.882). The effects of HAART on AIDS-related mortalities were affected by late diagnosis. Early detection of HIV infection and increased uptake of HAART are important for greater benefits in terms of lives saved.