  We study the effective field theory (EFT) description of the virtual particle
effects in quasi-single field inflation, which unifies the previous results on
large mass and large mixing cases. By using a horizon crossing approximation
and matching with known limits, approximate expressions for the power spectrum
and the spectral index are obtained. The error of the approximate solution is
within 10% in dominate parts of the parameter space, which corresponds to
less-than-0.1% error in the $n_s$-$r$ diagram. The quasi-single field
corrections on the $n_s$-$r$ diagram are plotted for a few inflation models.
Especially, the quasi-single field correction drives $m^2\phi^2$ inflation to
the best fit region on the $n_s$-$r$ diagram, with an amount of equilateral
non-Gaussianity which can be tested in future experiments.
