A half-wave plate (HWP) is a critical component for calibrating the gain ratio in polarization lidars. In this paper, the effects of a nonideal HWP on the gain ratio calibration measurements are analyzed. We focus on the ±45° method and the multi-rotation HWP method, which are the two main approaches for calibrating the gain ratio. Specifically, we discuss the influences of a nonideal HWP from two scenarios: an HWP with nonideal polarization properties and an ideal HWP under nonideal external conditions. The allowable ranges of relevant parameters for the nonideal polarization properties of HWPs are obtained, which can help to determine qualified HWPs in polarization lidars. Several external conditions, including ambient temperature variations, wavelength differences, and the tilt angles of HWPs, are also analyzed in detail. To the best of our knowledge, this paper represents the first comprehensive study on the effects of nonideal HWPs on the gain ratio calibration measurements, offering some guidelines on choosing a qualified HWP for proper use in polarization lidars.