  We propose an improved two-way continuous-variable quantum key distribution
(CV QKD) protocol by adding proper random noise on the receiver's homodyne
detection, the security of which is analysed against general collective
attacks. The simulation result under the collective entangling cloner attack
indicates that despite the correlation between two-way channels decreasing the
secret key rate relative to the uncorrelated channels slightly, the performance
of the two-way protocol is still far beyond that of the one-way protocols.
Importantly, the added noise in detection is beneficial for the secret key rate
and the tolerable excess noise of this two-way protocol. With the reasonable
reconciliation efficiency of 90%, the two-way CV QKD with added noise allows
the distribution of secret keys over 60 km fibre distance.
