## 1 Introduction  
Quantum key distribution (QKD) [1; 2; 3] is a primary application of quantum cryptography, which enables two remote legitimate communication parties, [PERSON] and [PERSON], to share random secret key sequences [4; 5; 6] in public channel preventing the eavesdropper [PERSON] by physical laws [7; 8; 9; 10]. There are two research fields for QKD, i.e., discrete-variable QKD(DVQKD) and continuous-variable QKD (CVQKD) [11; 12; 13; 14]. In DVQKD, information is encoded on properties of single photon, and it uses single-photon detector at the receiver side. In CVQKD, information is encoded in two quadratures of the optical field [15; 16], and relies on coherent detection including homodyne and heterodyne detectors [16; 17; 18]. In recent decades, CVQKD has become a hot issue due to its simplicity of implementation [19; 20], compatibility with modern fiber networks [21] and high detection efficiency that can realize higher secret key rate in short distances [22]. Unfortunately, the reconciliation efficiency of Gaussian modulated CVQKD (GM-CVQKD), whose secret keys are modulated following Gaussian distribution, is sensitive to channel losses [23], so that this scheme is unsuitable in long distance situations. To solve this problem, CVQKD with discrete modulation (DM-CVQKD) has been proposed by [PERSON] and [PERSON] [24]. In DM-CVQKD, such as four-state and eight-state modulation protocol, quantum states are nonorthogonal and information is represented by the sign of quadrature [25; 26; 27; 28], which is Gaussian random value in GM-CVQKD [29]. The maximum transmission distance of DM-CVQKD is greater than that of GM-CVQKD, and its unconditional security against collective attack have been proved [25] under the condition of low modulation variance. Actually, most studies of DM-CVQKD focus on one-way schemes that a local oscillator (LO) will be sent together with signal pulses to simplify the detection [30; 31]. However, the transfer of LO will open some loopholes for eavesdroppers. The loopholes can be exploited to conduct some attacks such as wavelength attacks, saturation attacks, and LO fluctuation attack [32; 33; 34]. An improved scheme called real local oscillator in which the LO is generated on the receiver's side without transmitting through the quantum channel has been proposed [35; 36; 37; 38]. In this scheme, the loopholes resulted by the transmission of LO are closed, but its performance would be poor because of the incidental frequency out-sync between two independent laser sources. Subsequently, plug-and-play (PP) dual-phase-modulated coherent-states (DPMCS) CVQKD have been proposed to solve the above-mentioned problems in experiments by [PERSON] et al. [39] and [PERSON] et al. [40]. In the PP DPMCS scheme, LO is generated by [PERSON] without transmitting through the quantum channel, whereas quantum state is modulated by [PERSON] based on the classical light sent from the source of [PERSON] [39]. Therefore, the loophole resulting from transmitting LO is closed, and the polarization drift of the fiber channel would be compensated automatically [41; 42]. However, the transmission of the classical light from Alice to Bob still results in extra excess noise [43] that would reduce the maximum transmission distance. Moreover, the PP protocol suffers Trojan-horse attack [44; 45] and the secure distance is limited by the Rayleigh scattering [46]. Fortunately, some non-Gaussian operations can be used for performance improvement of the CVQKD system because of the fact that this operation will strengthen the entanglement degree of the yielded system [47; 48; 49; 50]. Motivated by the characteristics of photon subtraction (PS) operation, which is a mature technology in implementation, in this paper, we propose an approach for enhancing the PP DM-CVQKD protocol with photon subtraction implemented at [PERSON]'s side.