We evaluated the isoline distribution of thermocline characteristics (the upper and lower boundary temperatures and depths) in the yellowfin tuna (Thunnus albacares) fishing grounds in the tropical Atlantic Ocean. We plotted the thermocline characteristics contour on a spatial overlay map using data collected on a monthly basis from Argo buoys and monthly CPUE (catch per unit effort) from yellowfin tuna longlines from the International Commission for the Conservation of Atlantic Tunas (ICCAT). In addition, frequency analysis and the empirical cumulative distribution function (ECDF) were used to calculate the optimum ranges for the thermocline characteristics of the central fishing grounds. Our analysis suggested that there were significant seasonal variations in the upper boundary temperature and depth of the thermocline in the central fishing grounds, which significantly influenced the temporal and spatial distribution of the yellowfin tuna population. However, the lower boundary temperature and depth of thermocline has little seasonal variation. The lower boundary depth of the thermocline isoline depth showed that the higher depth area (>250 m) was shaped like the letter ‘W’. Similarly, the lower boundary temperature of the thermocline isoline depth suggests that the temperature is higher besides the equator and lower around the equatorial Zonal. The overlay map suggest that the upper boundary of the thermocline in the fishing grounds of the yellowfin tuna ranges from 20 to 60 m in the equatorial zone during January to June. It is between 60 and 80 m from July to September, but increases to 100 m offshore of Namibia during July to September. The upper boundary of the thermocline in the fishing grounds decreases to about 60 m from October to December. The fishing grounds are distributed where the upper boundary temperature of the thermocline is between 26 and 29 ℃ all year in the equator zonal, whereas the hooking rates are low where the temperature falls below 24 ℃. The lower boundary depth of the thermocline of the fishing grounds in the equatorial zone ranges from 160 to 250 m and concentrates on 230 m. The lower boundary temperature of the thermocline in the fishing grounds is between 12 and 14 ℃ all year in the equatorial zone. Conversely, if the temperature is outside this range, the hooking rates are very low. The upper boundary of the temperature thermocline in the fishing grounds extends to 20 ℃ offshore of Namibia during July to November, and the lower boundary of the temperature and depth of the thermocline lies between 14 and 15 ℃ and 140 and 160 m, respectively. Frequency analysis and the empirical cumulative distribution function (ECDF) were used to calculate the optimum range of thermocline characteristics. The optimum depth range of the upper boundary temperature was 26-28.9 ℃.The optimum depth range of the lower boundary and the lower boundary temperature were 150-249 m and 12-14.9 ℃, respectively. The results were confirmed by using the Kolmogorov-Smirnov test. However, the upper boundary temperature was shown as insignificant from the statistical results. A preliminarily indication of the optimum distribution interval and seasonal change characteristics of each thermocline in the longline central fishing ground of yellowfin tuna was obtained in this paper. The conclusion could provide a reference for improving the efficiency of yellowfin tuna longline fishing and aid tuna resource management in the Atlantic Ocean.