  A main type of obstacles of practical applications of quantum key
distribution (QKD) network is various attacks on detection.
Measurement-device-independent QKD (MDIQKD) protocol is immune to all these
attacks and thus a strong candidate for network security. Recently, several
proof-of-principle demonstrations of MDIQKD have been performed. Although
novel, those experiments are implemented in the laboratory with secure key
rates less than 0.1 bps. Besides, they need manual calibration frequently to
maintain the system performance. These aspects render these demonstrations far
from practicability. Thus, justification is extremely crucial for practical
deployment into the field environment. Here, by developing an automatic
feedback MDIQKD system operated at a high clock rate, we perform a field test
via deployed fiber network of 30 km total length, achieving a 16.9 bps secure
key rate. The result lays the foundation for a global quantum network which can
shield from all the detection-side attacks.
