This article presents the synthesis and design of 3-D microwave absorber with 70° angular stability for C-band and X-band. The operating principle is firstly investigated, where the absorber is considered as an energy convertor. With the help of our proposed universal equivalent transmission line (TL) model, the absorptive performance can be accordingly synthesized. Then, a design method for efficient absorption under large angles is presented. By selecting a proper synthesized angle (SA), the angular stability can be effectively improved. After that, the prototype with 70° angular stability is designed as an example with structural realization and practical implementation. Measurements agree well with synthesized and simulated results, successfully verifying the proposed design method. For specific C-band and X-band applications, the measured average absorption ratios (ARs) under normal incidence, 45° incidence, and 70° incidence are 94.2%, 94.0%, and 92.3%. Minimum measured ARs within the operating bandwidth are 88.4%, 81.5%, and 82.0% for normal, 45°, and 70° incidences. Besides, the proposed absorber element owns the advantage of simple structure with only one resistor. Such a class of microwave absorber is a potential candidate for wide coverage electromagnetic absorption.