Time-tag error is an important error source of K-band ranging (KBR) system, and the present methods for time-tag error correction are dependent upon accurate measurement of inter-satellite time-tag offset. Aiming at the problem that currently the national measurement accuracy of inter-satellite time-tag offset cannot satisfy the demand for time-tag error correction, a novel correction method based on the combination of two optimal frequencies is proposed, and the quantity relation of the optimal frequencies is deduced. Research on the realization for combination of two optimal frequencies, which shows that it is unrealizable utilizing the existing KBR system, was performed. Consequently, a new KBR system adopting an advanced frequency flow was designed and realized. The experimental results demonstrate that the proposed method and the advanced KBR system effectively reduce the requirement for measurement accuracy of inter-satellite time-tag offset. 