Electrons scattering by plasmapheric hiss is the dominant mechanism that responsible for the formation of slot region (1.8 ≤ L ≤ 3) between the Earth’s inner and outer radiation belts. The cold plasma dispersion relation of plasmaspheric hiss is widely used to quantify its scattering effect on energetic electrons. However, the existence of hot plasmas in the realistic magnetospheric environment will modify the dispersion properties of plasmaspheric hiss. Based on Van Allen Probes observations, we select all hiss events in the slot region, and compare the observed hiss wave amplitudes with the converted hiss wave amplitudes deduced from cold plasma dispersion relation and electric field observations, and then study the dependence of the applicability of cold plasma dispersion relation of slot region hiss on spatial position and geomagnetic activity. The results show that the cold plasma approximation tends to overestimate the amplitude of slot region hiss. The differences between the observed amplitudes and the converted hiss wave amplitudes have a strong day night asymmetry, however, it shows slight dependence on the level of geomagnetic activities. In addition, we find that the converted wave magnetic field intensity is significantly lower (higher) than the observed magnetic field intensity at lower frequencies (higher frequencies), which indicates that cold plasma approximation generally overestimates (underestimates) the scattering effects of hiss waves on the lower (higher) energy electrons in the slot region. Our study confirms that the application scope of the cold plasma dispersion relation of slot hiss has strong spatial and frequency limitations, which is of great importance for us to deepen the understanding of the dynamic evolution of electrons in the slot region.