Potassium-boron-selenium fertilizer capable of promoting growth of fruit trees and degrading residual pesticide

The invention discloses a potassium-boron-selenium fertilizer capable of promoting the growth of fruit trees and degrading residual pesticide. Seleninic acid is neutralized by potassium hydroxide, so that potassium selenite is obtained; diboron trioxide is dissolved in water, and is mixed with the potassium selenite, and thereby potassium-boron-selenium reagent with the PH value between 7.0 and 8.8 is prepared. The potassium-boron-selenium fertilizer has the multiple fertilizer effects of selenium fertilizer and conventional potash fertilizer and boron fertilizer, and when the potassium-boron-selenium fertilizer is sprayed on the surfaces of the leaves of fruit trees, not only can the fruit-setting rate be increased, but also residual pesticide can be degraded and the selenium content of fruits can be increased.