An ultrasound system, comprising: an ultrasound data acquisition unit configured to transmit ultrasound signals to a target object and receive ultrasound echoes reflected from the target object to thereby acquire ultrasound data; and a processing unit configured to form a Doppler signal corresponding to each of a plurality of pixels constructing a Doppler mode image based on the ultrasound data, wherein the processing unit being further configured to adjust coefficients of a clutter filter for each of the pixels based on characteristics of the Doppler signal for each of the pixels for performing clutter filtering upon the Doppler signal, the coefficients including a cutoff frequency and stopband attenuation, characterized in that the processing unit is further configured to: determine minimum and maximum cutoff frequencies and minimum and maximum stopband attenuations based on the characteristics of the Doppler signal, determine a first coordinate having the minimum cutoff frequency and the minimum stopband attenuation and a second coordinate having the maximum cutoff frequency and the maximum stopband attenuation on a coordinate system associating the cutoff frequency with the stopband attenuation, and determine a path connecting the first coordinate and the second coordinate, wherein the processing unit is further configured to set a particular stopband attenuation and a particular cutoff frequency for a particular pixel according to amplitude of a clutter signal contained in the Doppler signal and the path connecting the first coordinate and the second coordinate. an ultrasound data acquisition unit configured to transmit ultrasound signals to a target object and receive ultrasound echoes reflected from the target object to thereby acquire ultrasound data; and a processing unit configured to form a Doppler signal corresponding to each of a plurality of pixels constructing a Doppler mode image based on the ultrasound data, wherein the processing unit being further configured to adjust coefficients of a clutter filter for each of the pixels based on characteristics of the Doppler signal for each of the pixels for performing clutter filtering upon the Doppler signal, the coefficients including a cutoff frequency and stopband attenuation, characterized in that the processing unit is further configured to: determine minimum and maximum cutoff frequencies and minimum and maximum stopband attenuations based on the characteristics of the Doppler signal, determine a first coordinate having the minimum cutoff frequency and the minimum stopband attenuation and a second coordinate having the maximum cutoff frequency and the maximum stopband attenuation on a coordinate system associating the cutoff frequency with the stopband attenuation, and determine a path connecting the first coordinate and the second coordinate, determine minimum and maximum cutoff frequencies and minimum and maximum stopband attenuations based on the characteristics of the Doppler signal, determine a first coordinate having the minimum cutoff frequency and the minimum stopband attenuation and a second coordinate having the maximum cutoff frequency and the maximum stopband attenuation on a coordinate system associating the cutoff frequency with the stopband attenuation, and determine a path connecting the first coordinate and the second coordinate, wherein the processing unit is further configured to set a particular stopband attenuation and a particular cutoff frequency for a particular pixel according to amplitude of a clutter signal contained in the Doppler signal and the path connecting the first coordinate and the second coordinate.