In this study, three oranges, tangerine(Clementine), orange(sweet orange) and pummelo(Wanbaiyou) were used raw material to identify EXP family members with bioinformatics methods. Their evolutionary development, structural characteristics, cis-acting elements, collinearity and the expression patterns of EXP in the process of juice sac expansion of ‘Qianyang bingtang orange’ and ‘Newhall navel orange’ were analyzed. The results revealed that 32, 30 and 26 dilatorin members were identified from tangerine(Clementine), orange(sweet orange) and pummelo(Wanbaiyou), respectively. The results of collinearity analysis showed that oranges, tangerines and pummelos contained 7 pairs, 8 pairs and 5 pairs of fragment repeats, respectively, while oranges and pomelo contained 1 pair of tandem repeats, respectively, speculating that fragment repeat events played a major role in the evolution of the EXP gene. The monitoring of sweet orange fruits at the expansion phase among different varieties showed that the individual juice sac volume of ‘Newhall navel orange’ was significantly larger than that of ‘Qianyang bingtang orange’. Furthermore, as the fruits expansion, the difference in individual juice sac volume gradually increased. Gene expression analysis showed that CsEXPA6, CsEXPA9, CsEXPA12 and CsEXPA15 were highly expressed in the juice sac tissues of orange fruits. The expression levels of CsEXPA6 and CsEXPA15 in the juice sacs of ‘Newhall navel orange’ were significantly higher than those of ‘Qianyang bingtang orange’ during the fruit expansion stage, and the expressions of CsEXPA6 and CsEXPA15 were significantly positively correlated with the increase of juice cell volume, suggesting that CsEXPA6 and CsEXPA15 might affect fruit size formation by regulating juice sac expansion.