The beak was the main feeding organ of cephalopods which contained abundant information of fishery biology and ecology. Based on the 1 009 Purple-back Flying Squid (Sthenoteuthis oualaniensis) samples which collected by Chinese lighting falling-net vessel in the northwest Indian Ocean from March to May 2019, the beak pigmentation was divided and determined. SPSS was used to conduct the ANCOVA test to determine whether gender studies were needed. The relationships between the beak pigmentation and the mantle length (L), body weight (W), sexual maturity and the external morphological parameters of the beak were analyzed by generating a box graph, the median values of the samples were fitted linearly. The results showed that the stage 2 was the dominant portion, accounting for 33.87% of the total samples during March to May, and the level of pigmentation stage increased with the time passed from March to May. There were significantly sexual differences in the relationships between beak pigmentation and mantle length, body weight, the external morphological parameters and sexual maturity (P < 0.05). The relationships between beak pigmentation and mantle length, body weight, the external morphological parameters of the beak (lower hood length, lower rostrum length, lower lateral wall length, lower wing length) were significantly relevant. The correlation coefficients (R2) between pigmentation stage and mantle length, body weight were 0.877 8, 0.863 5, 0.730 4 and 0.842 4 (Fig. 4). Under the same pigmentation stage, the range of mantle length and body weight of the males was smaller than that of females, indicating that females developed pigmentation faster than males. The minimum correlation coefficients (R2) between pigmentation stage and the external morphological parameters of the beak was 0.66 (Fig. 5) and his is higher than other cephalopods. The beak pigmentation stage increased with the increasing of sexual maturity (Table. 1). This study provides a basis for the fishery ecology research, effective exploitation and scientific management of S. oualaniensis resource based on the beaks.