Abstract: Cephalopod beak is an important hard tissue that records lots of growth information. In the Northwest Pacific Ocean, Enoploteuthis chunii is the prey of many marine mammals and fishes. Understanding phenotype changes in the growth of beak helps to know E. chunii ontogenetic trophic ecology variation and to investigate its ecological role in the system. In this study, the ontogenetic changes in beak phenotype were investigated by analyzing the variation of beak length and pigmentation of 91 squids, which were collected in the Northwest Pacific Ocean in March and September in 2019. AIC analysis shows that beak length and pigmentation of the squids had positive relationship with increasing mantle length. The beak length saltated in 31 and 41 mm. The upper crest length (UCL), lateral wall length (ULWL) and lower lateral wall length (LLWL) had high linear regression slope, which indicates that they grew faster than other beak parts. The proportion of pigmentation in the upper hood length (UHL), lower hood length (LHL) and LLWL saltated in 31 mm, but that in ULWL saltated in 41 mm. ANOVA analysis shows significant phenotypic changes of E. chunii beak (P<0.05). The study of ontogenetic phenotype changes such as size and pigmentation provides references for research on ontogenetic feeding shift.