  Quantum key distribution (QKD) allows two remote parties to share
information-theoretically secure keys. In recent years, a revolutionary
breakthrough called twin-field (TF) QKD has been developed to overcome the
linear key-rate constraint and greatly increases the achievable distance.
Phase-randomization and subsequent postselection play important roles in its
security proof. Later, no-phase-postselection TF-QKD was proposed and became a
popular variant, since the removal of phase postselection leads to a higher key
rate. However, the achievable distance is decreased compared to the original
one. Here, we propose a TF-QKD protocol with partial phase postselection.
Namely, its code mode is still free from global phase randomization and
postselection to make sure the advantage of the high key rate remains. On other
hand, phase postselection is introduced in the decoy mode to improve the
performance. Applying an operator dominance condition, we prove universal
security of the proposed protocol in the finite-key case under coherent
attacks, and numerical simulations confirm its potential advantages in terms of
key rate and achievable distance.
