Abstract: In distributed radar systems, the coherent synthesis gain during broadband transmit beamforming is severely degraded due to inconsistencies in frequency, time delay, and phase among transmitting nodes. To address this issue, this paper proposes a multi-tone-assisted coherent synthesis method for broadband signal transmission. To solve the problem of coupled coherent parameters (phase difference, time difference, and frequency difference) when the receiving node is unknown, the method first decouples the coherent parameters based on multi-tone signals transmitted by each node, enabling the estimation and compensation of phase, time, and frequency differences. Furthermore, to maintain coherent synthesis gain in dynamically changing radar scenarios, real-time estimation and compensation of coherent parameters are achieved through loop filtering. Simulation results demonstrate that the proposed method effectively estimates coherent parameters and maximizes coherent synthesis gain in dynamic environments. Moreover, the method is applicable to high-dynamic distributed radar systems for broadband coherent synthesis.