  A continuous-variable measurement-device-independent quantum key distribution
(CV-MDI QKD) protocol using squeezed states is proposed where the two
legitimate partners send Gaussian-modulated squeezed states to an untrusted
third party to realize the measurement. Security analysis shows that the
protocol can not only defend all detector side channels, but also attain higher
secret key rates than the coherent-state-based protocol. We also present a
method to improve the squeezed-state CV-MDI QKD protocol by adding proper
Gaussian noise to the reconciliation side. It is found that there is an optimal
added noise to optimize the performance of the protocol in terms of both key
rates and maximal transmission distances. The resulting protocol shows the
potential of long-distance secure communication using the CV-MDI QKD protocol.
