One member of staff and a person from a partner company died in the crash.
The incident took place on Wednesday at about 17:20 local time, the firm said. The people who died were inside the vehicle as it fell from the building.
Nio says it immediately started an investigation into the incident in co-operation with government officials.
The third storey area from where the car fell has been variously described as a showroom, a testing facility or a car park.
"Our company has collaborated with public security department to launch the investigation and analysis of the cause of the accident. Based on the analysis of the situation at the scene, we can initially confirm that this was an accident (not caused by the vehicle)," the company said in a statement.
"We feel very sad about this accident and would like to express our deepest condolences to our colleague and partner employee who lost their lives. A team has been set up to help the families," it added.