The work aimed to clarify the effects of metamitron as the fruit thinning agent on the apple fruit setting rate as well as the mineral nutrition, carbohydrates, and hormones of lateral fruitlets, thus providing a reference for applying chemical thinning technology of apples. Thirty 9-year-old apple trees (Tianhong 2/SH38/Malus micromalus) were used as test materials, and 300-ppm metamitron solution was sprayed 2 times when the diameter of the biggest lateral fruits were around 6 mm. Spraying water was used as the control. The setting rate of inflorescence and flower were investigated after physiological fruit drop. The mineral nutrition, carbohydrate, soluble protein, and hormone content of lateral fruits were measured after spraying 7, 9, 11, 17, and 29 d. The total nitrogen, phosphorus, and potassium were determined by semi-micro distillation, Mo-Sb-Vc colorimetry, and flame photometer, respectively. Glucose, fructose, sorbitol, sucrose, and hormone contents were determined by HPLC, while the soluble protein content was determined by Coomassie brilliant blue staining. The results showed that the fruit setting rate of inflorescence and flowers decreased by 17.95 and 27.63% compared with the control, respectively. The proportion of inflorescence with single fruit increased by 5.28 times in the treatment, while that of sitting three fruits and more decreased significantly, which was 47.91% of the control. The results of fruit setting rate and fruit setting ratio showed that metamitron could significantly reduce the fruit setting rate, and the single fruit rate was high, with the setting fruits distributed evenly. The total nitrogen content was higher than that of the control at all stages, and the difference between the control and the treatment was significant except 7 and 9 d. The total phosphorus and potassium contents of almost all treatments were significantly higher than those of the control. The total phosphorus contents of 11 and 29 d after spraying was 2.05 and 1.67 times of the control, and the total potassium content was 1.93 and 1.51 times of the control, respectively. The results of mineral nutrition showed that the fruit thinning effect of metamitron was not caused by the deficiency of inorganic nutrients. The content of the soluble protein decreased significantly compared with the control, and the content of each treatment period was 71.42, 72.37, 69.68, 64.20, and 86.02% of the control, respectively. The starch content of metamitron treatment was significantly higher than that of the control, with an increase of 5.57%-37.20%, yet the content of sucrose, glucose, fructose, and sorbitol decreased with different degrees. Therefore, the soluble carbohydrate content decreased significantly to 9.55%-52.57%. Soluble carbohydrates are the direct nutrients for the development of young fruits, so fruit thinning of metamitron may be caused by insufficient supply of soluble carbohydrates. The abscisic acid content generally increased after the treatment, and the ABA content in each period was 1.37-3.11 times that of the control. The ratio of (Z+GA3+IAA)/ABA decreased significantly, which could cause falling off of young fruits. As a result, metamitron has a good fruit thinning effect on apples, related to the decreased soluble protein content, an insufficient supply of soluble carbohydrate, increased ABA content, and decreased (Z+GA3+IAA)/ABA ratio.