A method of controlling speed change of a pruning machine including the steps of: pressing a power switch, setting up a power supply system, and sending an activating signal detecting voltage of the power supply system, and judging whether operation of the power supply system is maintained, sending a maintaining output signal and maintaining the voltage of the power supply system pressing a speed-regulating trigger button, converting a speed-regulating signal into an electrical signal for speed judgment, at the same time sending a driving signal after detection of a motor position signal, the driving signal being received by a three-phase bridge using a sampling resistor to convert a current into a voltage signal and inputting the voltage signal into the central processor, and comparing a set current value according to the input voltage signal, thereby controlling the motor to rotate according to a certain proportion, or a maximum power.
