The existing heterodyne power combiners are not suitable for applications where input and output signals need to be in the same direction with limited space. To solve the problem, this paper designs a high-power and miniaturized heterodyne power combiner operating at frequencies of 9.3 GHz and 9.7 GHz. Based on the traditional filter-based heterodyne power combiner, the proposed design utilizes an over-mode rectangular waveguide E-plane power combiner. The waveguide filters are parallel and the input ports are also located on the same plane, so that the combiner is suitable for the specific applications. The sizes of the rectangular waveguide are reduced to suppress higher-order modes. Besides, the distance between mode strips is decreased in integer multiples of half-wavelength of the waveguide to compresses the overall length with high power capacity. The combiner has a length of 9.2λ, a width of 1.5λ and a height of 2.8λ, while λ is the wavelength corresponding to the frequency of 9.5 GHz in free space. At 9.3 GHz and 9.7 GHz, the return loss of the combiner is more than 20 dB, its combining efficiency is more than 98% , and the isolation between input ports is more than 20 dB. At microwave pulse breakdown threshold of 80 MV/m, the combiner provides a power capacity of 310 MW.