The invention relates to a method for utilizing melatonin for promoting facility tomato fruit setting and growth.A melatonin solution with the concentration being 100 micromole/L is sprayed to the whole inflorescence of tomatoes growing in a low-temperature environment.The method is easy to implement, easy to operate, low in investment cost, obvious in effect, small in toxic and side effect, and capable of remarkably increasing the fruit setting rate and the fruit growing speed of the tomatoes in out-of-season facility production, so that economic benefits of the out-of-season facility tomato production are increased.