  We studied the linear electro-optic effect of chiral topological semimetal
RhSi which is characterized by high-fold chiral fermions separated in energy
space. We identify that the general second order conductivity
$\sigma^{(2)}_{xyz}(\omega=\omega_{1}+\omega_{2};\omega_{1},\omega_{2})$
includes a real symmetric component and an imaginary antisymmetric component,
which are from the inter-band shift and intra-band injection current with
frequency $\omega$, respectively. The $\sigma^{(2)}_{xyz}$ is significantly
enhanced by the high electron velocity and nontrivial band topology of chiral
fermion, and modifies the phase velocity of light wave. We also predict that
the electro-optic coefficient $\chi_{xyz}^{(2)}(\omega;\omega,0)$ of chiral
crystal RhSi is about 7000 pm/V at photon energy 0.01 eV and 1.1 eV, which is
about 200 times that of widely used LiNbO$_{3}$ crystal. The giant
electro-optic coefficient renders a relatively low half-wave voltage in order
of hundreds volt, and demonstrates potential application as electro-optic
crystals for the wavelength in the second telecom window of optical fiber
communications.
