A grating-type ultra-broad band far infrared absorber based on three kinds of semiconductor materials: indium phosphide (InP), indium arsenide (InAs) and indium antimonide (InSb) is designed. The groove depth of the absorber is changed gradually. Its absorption mechanism is surface plasma resonance and dielectric cavity resonance. They are calculated by a finite-difference frequency-domain (FDFD) method. The calculation results show that the change of the number of the groove has a relatively great influence on the absorptivity of the absorber while the changes of groove depth, groove width, coating thickness and grating period has a less influence on the absorptivity of the absorber. Under the condition of optimized structural parameters, the average absorptivity of the absorber is greater than 92% in the incident wavelength range from 28 to 75 m at the incident angle from 0 to 80 degree. The absorber designed is expected to find applications in far infrared detection, etc.