A viscoelasticity measurement method and an ultrasonic imaging system. The method includes: generating a shear wave that propagates in a target area of a measured object; emitting, to the target area, an ultrasonic wave that tracks the shear wave, and receiving an ultrasonic echo returned from the target area, so as to obtain ultrasonic echo data; obtaining tissue motion information in a shear wave propagation process according to the ultrasonic echo data; extracting, from the tissue motion information, parts of tissue motion information corresponding to shear waves of at least two different frequencies; and outputting the parts of tissue motion information of the shear waves of the at least two different frequencies. The parts of tissue motion information are used for reflecting a viscosity feature of the target area.