The state-run Beijing News says Zheng Zhibiao argued at his trial in early December that he had been concerned over a lack of protection of artifacts held at the former palace of China's emperors.
The vast network of ceremonial halls, imperial apartments and other buildings is Beijing's premier tourist attraction.
Beijing News says Zheng, a display designer at the museum's exhibition division, had complained about a lack of professionalism in protecting and renovating the museum.