Fruit in general or pineapple in particular, when frozen, have undesirable changes in quality. For example, the structure is no longer as rigid as the original, the loss of dry matter as well as the loss of vitamin C is significant, etc. Therefore, finding and choosing a method to ensure the quality of frozen fruit is also economical is a matter of necessity. Accordingly, the influence of pretreatment methods on the quality of frozen Cau Duc pineapple (Ananas comosus) was investigated. Research results show that when soaking Cau Duc pineapple samples at a sugar concentration of 500 g/L for 60 min, the quality of frozen samples will be the highest. Cau Duc pineapple is pretreated (blanched) in 0.15% CaCl2 solution at 55°C, for 10 min, then soaked for 60 min in sucrose solution (500 g/L) to maintain hardness of the sample about 50% compared with fresh sample. At the same time, in this pretreatment mode, the dry matter and vitamin C content in frozen sample, after thawing, also lose the least.