  Underlying mixing of scalar mesons is studied in $\eta'\rightarrow
\eta\pi\pi$ decay within a generalized linear sigma model of low-energy QCD
which contains two nonets of scalar mesons and two nonets of pseudoscalar
mesons (a quark-antiquark nonet and a four quark nonet). The model has been
previously employed in various investigations of the underlying mixings among
scalar mesons below and above 1 GeV (as well as those of their pseudoscalar
chiral partners) and has provided a coherent global picture for the physical
properties and quark substructure of these states. It is found that the linear
sigma model with only a single lowest-lying nonet is not accurate in predicting
the decay width, but inclusion of the mixing of this nonet with the
next-to-lowest lying nonet, together with the effect of final state interaction
of pions, significantly improves this prediction and agrees with experiment up
to about 1\%. It is also shown that while the prediction of the leading order
of the generalized model for the Dalitz parameters is not close to the
experiment, the model is able to give a reasonable prediction of the energy
dependencies of the normalized decay amplitude squared and that this is
expected to improve with further refinement of the complicated underlying
mixings. Overall this investigation provides further support for the global
picture of scalar mesons: those below 1 GeV are predominantly four-quark states
and significantly mix with those above 1 GeV which are closer to the
conventional p-wave quark-antiquark states.
