Based on coaxial-waveguide radial combiner, a broadband arbitrary-way power combiner in the terahertz band is proposed. In this model, the second order ladder transforming structure is utilized to change the direction of the electric field from the coaxial to the radial direction, and an equally distributed Y-junction is adopted to realize the arbitrary-way dividing. A 5-way coaxial-waveguide combiner is implemented in D band for testing. The back-to-back measurement results show that the return loss and the insertion loss within the bandwidth from 130 GHz to 150 GHz are less than 15 dB and 2 dB, respectively. Thus, the transmission loss of the combiner is about 0.72 dB and the combining efficiency is 84.7%.