The purpleback squid () is widely distributed in the South China Sea and northwestern Indian Ocean, with significant diurnal vertical movement. Therefore, it is very important to study the relationship between the catch of , the surface temperature and the vertical temperature structure. Based on catch data from a light-falling net fishing vessel and temperature data in the South China Sea from September to October 2012 and March to April 2013, the relationships among the catch per net of , surface temperature and vertical temperature structure were studied. The degree of gray incidence was used to confirm the greatest factors affecting the catch of . The results indicated that spring was the best fishing period for ; the total catch and the catch per net were obviously higher than that in autumn. Thus there was obvious seasonal variation in the fishing grounds of . The catch was mainly concentrated in the waters near 10°-15°N, 111°-117°E in spring, and in the waters near 13°-15°N, 117°-118°E in autumn. The suitable sea surface temperature (SST) in the fishing grounds was 25.6-29.6℃ in spring and 27.6-30.0℃ in autumn; however, the optimum SST of was the same at 28.5-29.5℃ in spring and autumn. The temperature gradient in the fishing grounds was different between spring and autumn when choosing the S15 and S22 sites as representative sites. A thermoclineboth in spring and autumn was formed in the water layer at 50-100 m, and the thermocline intensity in spring was higher than in autumn. The catch per net of decreased with the increase in the temperature gradient from 5 m to 50 m in spring, and the catch per net in the class interval of 0.00-0.05℃/m was higher than the others. By contrast, the catch per net of increased with the increase in the temperature gradient from 5 m to 50 m in autumn, and the catch per net in the class interval of 0.15-0.20℃/m was higher than the others. The degree of gray incidence (0.84) indicated that the temperature gradient from 5 m to 50 m was the greatest factor affecting the catch of , followed by the latitude, temperature gradient from 5 m to 100 m, SST and longitude.