  Channel loss seems to be the most severe limitation on the practical
application of long distance quantum key distribution. The idea of twin-field
quantum key distribution can improve the key rate from the linear scale of
channel loss in the traditional decoy-state method to the square root scale of
the channel transmittance. However, the technical demanding is rather tough
because it requests single photon level interference of two remote independent
lasers. Here, we adopt the technology developed in the frequency and time
transfer to lock two independent lasers' wavelengths and utilize additional
phase reference light to estimate and compensate the fiber fluctuation. Further
with a single photon detector with high detection rate, we demonstrate twin
field quantum key distribution through the sending-or-not-sending protocol with
realistic phase drift over 300 km optical fiber spools. We calculate the secure
key rates with finite size effect. The secure key rate at 300 km
($1.96\times10^{-6}$) is higher than that of the repeaterless secret key
capacity ($8.64\times10^{-7}$).
