BACKGROUND: In Shenzhen, the Emergency Medical Service (EMS) system has been in service since 1997. This study aims to examine the operation of Shenzhen 120 EMS center and to identify the reasons of calling EMS. METHODS: In this retrospective quantitative descriptive study, the data from the Shenzhen 120 EMS registry in 2011 were analyzed. RESULTS: Shenzhen 120 EMS center is a communication command center. When the number of 120 are dialed, it is forwarded to the closest appropriate hospital for ambulance dispatch. In 2011, the Shenzhen 120 EMS center received 153160 ambulance calls, with an average of 420 calls per day. Calling emergency services was mainly due to traffic accidents. Trauma and other acute diseases constituted a majority of ambulance transports. The adult patients aged 15–60 years are the principal users of EMS. There are no recognized 'paramedic' doctors and nurses. The pre-hospital emergency service is under the operation of emergency departments of hospitals. Shenzhen at present does not have specialized pre-hospital training for doctors and nurses in post-trauma management. Moreover, specialized pre-hospital training, financial support, and public health education on proper use of EMS should be emphasized. CONCLUSION: The Shenzhen 120 EMS center has its own epidemiology characteristics. Traumatic injury and traffic accident are the main reasons for calling ambulance service. In-depth study emphasizing the distribution and characteristics of trauma patients is crucial to the future development of EMS.