  The measurement-device-independent quantum key distribution (MDI-QKD)
protocol plays an important role in quantum communications due to its high
level of security and practicability. It can be immune to all side-channel
attacks directed on the detecting devices. However, the protocol still contains
strict requirements during state preparation in most existing MDI-QKD schemes,
e.g., perfect state preparation or perfectly characterized sources, which are
very hard to realize in practice. In this letter, we investigate
uncharacterized MDI-QKD by utilizing a three-state method, greatly reducing the
finite-size effect. The only requirement for state preparation is that the
state are prepared in a bidimensional Hilbert space. Furthermore, a
proof-of-principle demonstration over a 170 km transmission distance is
achieved, representing the longest transmission distance under the same
security level on record.
