Tuna purse seining is the most efficient fishing method in the modern tuna fisheries, this present study analyzed the sinking performance in tuna purse seines and its relationships with impact factors, which are beneficial to improve efficient fishing in tuna purse seines. Based on the data collected from the tuna purse seiner “JINHUI 7” operating in the Western and Central Pacific Ocean (WCPO) during September to December 2011, the present study analyzed the relationship between sinking depth (H) and other variables, including shooting duration (T), shooting velocity (V0), length of purse line (L) , length of ran line (L1) and current speed (V10, V60 and V120) of 10m, 60m and 120m layers, the impact factors on sinking depth are also estimated using the generalized additive model (GAM). The results show that (1) the relationship between the sinking depth (H) and time (t) for the middle section of leadline can be described by the formula H = -0.0002 t2 0.4086t 1.8099 (R2 = 0.9993). (2) the sinking speed decrease with the increasing depth, the relationship between the sinking speed (V) at the middle part of leadline and time (t) can be described using the formula V = 2.5 × 10-7 t2 - 6 × 10-4t 0.4412 (R2 = 0.9852). (3) the analysis of stepwise GAM shows that four factors have a significant impact on the sinking depth (P < 0.05) and the significance can be ordered by V120, L ,T and V60; (4) the analysis of GAM shows that the sinking depth increases gradually with the increasing shooting duration, which concentrates on the period of 500-550s; current speed has a negative correlation to the sinking depth; the length of purse line is mainly concentrated in about 1800m.