  Rapidly and randomly drifted reference frames will shorten the link distance
and decrease the secure key rate of realistic quantum key distribution (QKD)
systems. However, an actively or inappropriately implemented calibration scheme
will increase complexity of the systems and may open security loopholes. In
this article, we present a free-running reference-frame-independent (RFI) QKD
scheme, where measurement events are classified into multiple slices with the
same misalignment variation of reference frames and each slice performs the
post-processing procedure individually. We perform the free-running RFI QKD
experiment with a fiber link of 100km and the misalignment of the reference
frame between Alice and Bob is varied more than 29 periods in a 50.7-hour
experiment test. The average secure key rate is about 734 bps with a total loss
of 31.5 dB, which achieves the state-of-art performance of the long-distance
RFI QKD implementations. Our free-running RFI scheme can be efficiently adapted
into the satellite-to-ground and drone based mobile communication scenarios, as
it can be performed with rapidly varying reference frame and a loss more than
40 dB, where no secure key can be obtained by the original RFI scheme.
