According to the present disclosure, there is provided a method for controlling an RPM of an engine of construction machinery, including and converting and setting the received load factor to a load variation rate; receiving a current load factor of the engine and converting the received current load factor of the engine to a load variation rate; controlling driving of the engine with a corrected RPM, which is obtained by decreasing the RPM of the engine by a predetermined rate from a preset RPM when the current load factor of the engine and the load variation rate according to the current load factor are equal to or smaller than a predetermined rate of the predetermined load factor of the engine and a predetermined rate of the load variation rate for a first time, respectively; and controlling continuously decreasing the corrected RPM to a predetermined rate or smaller, when the current load factor of the engine and the load variation rate according to the current load factor are equal to or smaller than the predetermined rate of the predetermined load factor of the engine and the predetermined rate of the load variation rate for a second time, respectively.