In order to study the effect of cage cultivation of large-size Thamnaconus septentrionalis Günther, with offshore small cage as cultivation platform, Thamnaconus septentrionalis Günther of 1.5 month was taken as a sample for 50 days of cultivation experiment at water temperature of 20.0℃-26.0℃. The results showed that: 1) The small-size fry with initial average body mass of 2.6 g and average total length of 5.4 cm has grown to large-size fry with average body mass of 41.10 g and average total length of 15.10 cm, with average daily weight increase of 0.75 g. 2)There was a linear relationship between the total length and the day-age: L = 0.201 5x + 5.244 5(R² = 0.996 4). There was a power exponent relationship between the body mass and the day-age: W = 3.151 8 e0.053 8x(R² = 0.982 4), with instantaneous rate of average body mass increase of 0.05%. 3. There was an exponential relationship between the total length and the body mass, with the regression equation: W = 0.770 5e0.267 8L(R2 = 0.991 8), indicating that the body mass was positively related to the total length, which was the geometric growth type. 4. The body mass increase and the instantaneous rate of body mass increase of the sample were affected much by the temperature, and the weight increased quickly at 20.0℃-23.0℃. The study indicated that small-size Thamnaconus septentrionalis Günther grew fast, was fed efficiently, got less sick and was easy to be managed, so the cultivated fry could be used to solve the shortage of large-size fry cultivated in large cage. Offshore cage cultivation of large-size Thamnaconus septentrionalis Günther is an important bridge for its cultivation in large cage or deep sea.