  The optical phase shifter that constantly rotates the local oscillator phase
is a necessity in continuous-variable quantum key distribution systems with
heterodyne detection. In previous experimental implementations, the optical
phase shifter is generally regarded as an ideal passive optical device that
perfectly rotates the phase of the electromagnetic wave of $90^\circ$. However,
the optical phase shifter in practice introduces imperfections, mainly the
measurement angular error, which inevitably deteriorates the security of the
practical systems. Here, we will give a concrete interpretation of measurement
angular error in practical systems and the corresponding entanglement-based
description. Subsequently, from the parameter estimation, we deduce the
overestimated excess noise and the underestimated transmittance, which lead to
a reduction in the final secret key rate. Simultaneously, we propose an
estimation method of the measurement angular error. Next, the practical
security analysis is provided in detail, and the effect of the measurement
angular error and its corresponding compensation scheme are demonstrated. We
conclude that measurement angular error severely degrades the security, but the
proposed calibration and compensation method can significantly help improve the
performance of the practical CV-QKD systems.
