In this paper, a new method to design multi-way in-phase feeding networks with arbitrary power distribution is proposed based on a circular substrate integrated waveguide (SIW) cavity. The electric-field distribution of the TE101 mode in a circular SIW cavity is inspected to guide the design. Analysis results show that arbitrary power splitting ratio can be achieved by arranging output feeding points at proper positions. In order to verify the design concept, an eight-way SIW feeding network with different power distribution is designed. Finally, measured results, agreeing well with simulated ones, is provided and show that the proposed network operates at 11.52 GHz with a 10 dB bandwidth of 13.2%. The networks in this paper can serve as a good candidate for low-sidelobe antenna arrays application.