This study presents the design of a triple-band metamaterial absorber with a composite structure that can fully absorb the incident terahertz radiation at a certain frequency. The absorption remains high at incident angles up to 50°. The conditions for perfect absorption and the frequency shift caused by the permittivity of the dielectric spacer were analyzed via the interference theory. Further, the effect of the length of the Jerusalem cross on absorption was analyzed by applying the transmission line theory combined with the interference theory. With the increase of the short edge length, the absorption peak redshifts. The experimental results are consistent with those from the simulation, the interference theory, and the transmission line theory, thus the results here provide guidance for the design of a metamaterial absorber.