The present invention relates to a method for microwave-assisted extraction of bioactive compounds from plant part, comprising the steps of: immersing the plant part in an extraction solvent at a predetermined solvent to feed ratio to form a sample mixture; and irradiating the sample mixture in a closed microwave system at a predetermined power for a predetermined duration under heating modes selected from constant power mode, multi-steps power mode, intermittent power mode and constant temperature mode, wherein the predetermined power rely on actual absorbed power and absorbed energy irradiated during diffusion step of the microwave-assisted extraction.