Dewar is an essential part of the large format IR detector assembly，which provides the necessary hermetic vacuum，the cold operating temperature and the interfaces of optics，electronics，and mechanics.So the Dewar must have high inherent reliability.However，the samples of Dewar for large format IR detector are limited and the traditional probabilistic reliability design method is restricted.In this paper，non-probabilistic reliability model is established by utilizing interval variables analysis.Based on this non-probabilistic reliability model，the reliability optimization design method is applied in the structure design of the large format IR detector Dewar.It indicates that this non-probabilistic reliability optimization design does not require a presumed probability distribution of the uncertain parameters and only the bounds or ranges of variation of them are required.So it is a good method when the available design data of uncertainties is less.