In the framework of the upgrade of the SPARC_LAB facility at INFN-LNF, named EuPRAXIA@SPARC_LAB, a high gradient linac is foreseen. One of the most suitable options is to realize it in X-band. The EuPRAXIA@SPARC_LAB linac layout is based on an S-band Gun, three S-band TW structures and an X-band booster with a bunch compressor. In this presentation it is illustrated the preliminary RF design of the X-band booster. It is based on 0.5 m long traveling wave accelerating structures operating in the 2π/3 mode and fed by klystrons and pulse compressor systems.The design has been driven by the need of a high accelerating gradient required to achieve a high facility compactness, which is one of the main goals of the EuPRAXIA project. The single cell parameters have been calculated by electromagnetic simulations and, on the basis of these results, the accelerating structure length and geometry have been optimized by numerical studies. Finally, the basic RF power distribution layout has been designed.