  Among the various versions of the twin-field quantum key distribution
(TF-QKD) protocol [M.Lucamarini, Z. Yuan, J. Dynes, and A. Shields, Nature
(London) 557, 400 (2018)] that can overcome the rate-distance limit, the TF-QKD
without phase postselection proposed by Cui et al. [Phys. Rev. Appl. 11, 034053
(2019)] is an elegant TF-QKD that can provide high key rates since the
postselection of global phases has been removed. However, the achievable
distance of this variant is shorter than that of the original phase-matching
QKD [X. Ma, P. Zeng, and H. Zhou, Phys. Rev. X 8, 031043 (2018)]. In this
paper, we propose a method for improving its performance by introducing an
additional decoy mode. The upper bound of the information leakage can be more
tightly estimated; hence, both the key rate and the achievable distance are
significantly improved. Interestingly, the operation of the proposed additional
decoy mode is the same as that of the code mode; hence, it does not introduce
difficulties into the experimental system. In addition, the improvement is
substantial with finite decoy states, which is meaningful in practice.
