To address the complex beam control in traditional multiple-input multiple-output (MIMO) systems, researchers have proposed adaptive beam alignment using retro-directive antenna (RDA) arrays. This approach creates echo resonance between the base station (BS) and user equipment (UE), significantly reducing computational load. However, conventional resonant beam systems (RBS) suffer from echo interference due to the shared uplink and downlink frequency. Therefore, this paper proposes an innovative resonance beam-based integrated sensing and communication (RB-ISAC) system designed for efficient passive sensing and bidirectional communication. In this system, the UE operates passively, with both the BS and UE utilizing a phase conjugation and frequency conversion structure to decouple uplink and downlink carrier frequencies, ensuring continuous electromagnetic wave oscillation between the two ends. Effective compensation for signal propagation loss enables resonance after multiple oscillations. At this point, the beam's field forms a low-diffraction-loss, highly focused pattern, automatically aligning the transmitter and receiver. This enables high-precision passive positioning alongside robust uplink and downlink communication. Simulation results demonstrate the proposed system achieves resonance within multiple iterations, supporting uplink and downlink communication up to 5 m, and enabling passive direction of arrival (DOA) estimation with an error under 2$^\circ$ .