Continuous-variable quantum key distribution is a practical solution for secure communication, compatible with classical optical networks. The discrete modulation scheme simplifies implementation but complicates security analysis, where the shot-noise unit is a critical parameter. Traditional two-step shot-noise unit calibration is experimentally cumbersome and unsuitable for real-time systems. Here, we propose a one-time shot-noise unit calibration method for discrete-modulated continuous-variable quantum key distribution, which determines the shot-noise unit in a single step. Our approach introduces a revised detector model that treats electrical noise loss as untrusted, incorporating it into the channel. We establish the equivalence between the entanglement-based and prepare-and-measure schemes under this model and derive the corresponding positive operator-valued measure for heterodyne detection. Combining trusted-source-noise model, a complete practical security model can be achieved. Security analysis and simulations show that the proposed method achieves performance close to traditional protocols, facilitating the deployment of discrete-modulated continuous-variable quantum key distribution systems with automated, real-time calibration.

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• Quantum communication
• Quantum communication, protocols & technology