Only two isospin-singlet scalar mesons $f_0(600)$ ($sigma$) and $f_0(980)$ exist below 1 GeV, so that it is natural to suppose that they are two energy eigenstates which are mixtures of ${1oversqrt 2}(ubar u+dbar d)$ and $sbar s$. Is this picture right? Generally, it is considered that $f_0(600)$ mainly consists of ${1oversqrt 2}(ubar u+dbar d)$, if so, the dominant component of $f_0(980)$ should be $sbar s$. The recent measurement of the CLEO collaboration on the branching ratio of $D_sto f_0(980) e^+ u_e$ provides an excellent opportunity to testify the structure of $f_0(980)$, namely whether the data can be understood as long as it consists of mainly the conventional $qbar q$ structure. We calculate the form factors of $D_sto f_0(980)$ in the light-front quark model (LFQM) and the corresponding branching ratio of the semileptonic decay. By fitting the data, we obtain the mixing angle $phi$. The obtained mixing angle shows that the $sbar s$ component in $f_0(980)$ may not be dominant.
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