Abstract: As a hard tissue of cephalopods supporting internal organs and mantle, the gladius grows throughout the entire life cycle and is irreversible, which is a good carrier for recording the complete life history information of cephalopods. To explore the factors affecting the external morphology of gladius of Sthenoteuthis oualaniensis, the morphological growth characteristics of gladius structure were studied based on the 1388 samples collected in the northwest Indian Ocean in the autumn of 2019 (September to November), we analyzed the influencing factors of the morphology change of the gladius. The results showed that gladius length (GL), cones length (CL), vanes length (VL), greatest width of cones (GWC) and greatest width of proostracum (GWP) could be used as characteristic parameters to describe the change in the shape of the gladius. There were significant differences in the morphologic growth between genders, different gonad maturity stages, and different mantle length (P<0.05). There was no significant difference in VL/GL values between different mantle length groups (P>0.05), but there was significant difference among different gonad maturity (P<0.05). However, GWC/GWP values were significantly different between different mantle length groups and gonad maturity (P<0.05). The results showed that gonadal maturity stages Ⅱ and Ⅲ may be the inflection points of the changes of the morphology of gladius of S. oualaniensis. In the process of individual growth, the growth rate of each structure of gladius length was basically the same, while the growth rate of cones was slower than that of proostracum. The study of the external morphological influencing factors of the gladius of S. oualaniensis provides a scientific reference for the subsequent study of fishery biology and ecology, and can also provide a scientific basis for the use of hard tissue to divide and identify cephalopods.