Abstract: In order to study the effects of different melatonin treatments on the growth and development of weaned lambs, 50 three-month-old male multifetal fine-wool lambs with similar body weights were randomly selected and divided into 5 groups: control group（no exogenous melatonin stimulation）, experimental group 1（one-time subcutaneous implantation of 30 mg/only melatonin sustained-release granules in the back）, experimental group 2（subcutaneous implantation 10 mg/only melatonin sustained-release granules in the back, once every 30 days during the test period）, experimental group 3（one-time intramuscular injection of 30 mg/only melatonin gel）, and experimental group 4（intramuscular injection 10 mg/only of melatonin gel, once every 30 days during the test period）.There were 10 animals in each group.The pre-test period of 10 days, and the test period was 90 days.The body weight and blood melaton in content were measured before the test（day 0） and every 30 days, and the daily weight gain was calculated.Body size indexes（body height, body length, chest circumference, tube circumference, waist angle width） were measured at the beginning and end of the experiment, and the effects of exogenous melatonin on body weight and body size of experimental sheep were analyzed, as well as the correlation between melatonin content and body weight. The results showed that at the end of experiment, the body weight and daily gain in the experimental group 2 were significantly higher than those in the other four groups（P＜0.05）; the body heights in the control group and the experimental group 3 were significantly higher than those in the other three groups（P＜0.05）; the body length in the experimental group 1 and the experimental group 4 was significantly higher than that in the other three groups（P＜0.05）,and the chest circumference in the experimental group 2 was significantly higher than that in the other four groups（P＜0.05）, there was no significant difference in the tube circumference and waist angle width of the 6-month-old sheep among the five groups（P＞0.05）. On the 30 th day, the melatonin content of the multiple parity fine wool sheep in the experimental group 4 was significantly higher than that in the other four groups（P＜0.05）.On the 60 th day, the melatonin content in the control group and the experimental group 3 was significantly higher than that in the other three groups（P＜0.05）. On the 90 th day, the melatonin content in the experimental group 1 and the experimental group 2 was significantly higher than that in the other three groups（P＜0.05）, and the content in the test group 2 was the highest, with an average melatonin content of 29.23 ng/mL. The content of melatonin was significantly correlated with the weight of multiple litters of fine wool sheep（P＜0.01）.The results indicated that giving an appropriate amount of exogenous melatonin could promote the growth and development of multiple fine-wool lambs, but it was related to the way of treatments. In contrast, the way of implanting 10 mg/only melatonin sustained-release granules, and 3 times was the best.