  We point out the $CP$ asymmetry in the chain decay of $\Lambda_c^+\to
\Delta^+K(t)(\to \pi^+\pi^-)$ is determined by the branching fractions of
$\Lambda^+_c\to \Delta^{++} K^-$, $\Lambda^+_c\to \Delta^{+} K^0_{S,L}$ and
$\Lambda^+_c\to \Delta^{0} K^+$ modes under the isospin symmetry. The
ambiguities from loop-induced quantities and $SU(3)_F$ breaking hadronic
effects are avoided. Once the $CP$ asymmetry in the $\Lambda_c^+$ decays into
$\Delta^+$ and neutral kaons is confirmed by experiments, we can check if it is
beyond the Standard Model, or verify the $CP$-violating effect resulted from
the interference between the Cabibbo-favored and the doubly Cabibbo-suppressed
amplitudes with the neutral kaon mixing. Future measurements of branching
fractions play a critical role in reducing the uncertainties.
