  The power spectrum of primordial tensor perturbations $\mathcal{P}_t$
increases rapidly in high frequency region if the spectral index $n_t>0$. It is
shown that the amplitude of relic gravitational wave $h_t$($5\times10^9$Hz)
varies from $10^{-36}$ to $10^{-25}$ while $n_t$ varies from $-6.25\times
10^{-3}$ to $0.87$. High frequency gravitational waves detector that is
proposed by F.-Y. Li detects gravitational waves through observing the
perturbed photon flux that is generated by interaction between the relic
gravitational waves and electromagnetic system. It is shown that the
perturbative photon flux $N_x^1$($5\times10^9$Hz) varies from
$1.40\times10^{-4}\rm s^{-1}$ to $2.85\times10^{7}\rm s^{-1}$ while $n_t$
varies from $-6.25\times 10^{-3}$ to $0.87$. Correspondingly, the ratio of the
transverse perturbative photon flux $N_x^1$ to the background photon flux
varies from $10^{-28}$ to $10^{-16}$.
