Abstract: In the Forbidden City, the last process of roof construction of an ancient building is that a ceremony named "Helong" is held. In the ceremony, a worker buries a costful box in the middle of the ridge. The box is filled with "Zhenwu", which means goods for exorcism or blessing. To effectively protect Chinese ancient buildings, the costful box, Zhenwu and ceremony of Helong are studied based on engineering practice and historical literature. Results show that material and size of costful box are different for different buildings; the middle of ridge location relates closely to Chinese ancient ideologies of "harmony between nature and human being" and "middle location for winner"; the number of each kind of Zhenwu is usually five, reflecting that "five" meant large amount for ancient Chinese people; the Helong ceremony belongs to Chinese ancient sacrifice culture, the selection of unmarried male worker relates closely to Chinese ancient ideologies such as "education under god", "expelling evil by Yangqi" and so on.