A compact interdigital H-mode drift-tube linac (IH-DTL) with the alteating-phase-focusing (APF) method, working at 325 MHz was designed for an injector of a proton medical accelerator. When fed in with a proper RF (radio frequency) power, the DTL cavity could estab-lish the corresponding electromagnetic field to accelerate the‘‘proton bunches’’from an input energy of 3 MeV to an output energy of 7 MeV successfully, without any addi-tional radial focusing elements. The gap-voltage distribu-tion which was obtained from the CST?Microwave Studio software simulations of the axial electric field was com-pared with that from the beam dynamics, and the errors met the requirements within ± 5%. In this paper, the RF design procedure and key results of the APF IH-DTL, which include the main RF characteristics of the cavity, frequency sensitivities of the tuners, and coupling factor of the RF power input coupler are presented.