  The photon statistics of a quantum key distribution (QKD) source is crucial
for security analysis. In this paper, we propose a practical method, with only
a beam splitter and photodetector involved, to monitor the photon statistics of
a QKD source. By implementing in a Plug&Play QKD system, we show that the
method is highly practical. The final secure key rate is 52 bit/s, comparing to
78 bit/s when the source is treated as a trusted source.
