  We investigate the tachyonic instability of Kerr-Newman (KN) black holes in
the Einstein-Maxwell-scalar (EMS) theory with a positive coupling parameter
$\alpha$. This corresponds to exploring the onset of spontaneous scalarization
for KN black holes. For this purpose, we use the hyperboloidal foliation method
(HFM) to solve the linearized scalar equation numerically. We obtain a 3D graph
[$\log_{10}\alpha(a,Q)$] which indicates the onset surface for spontaneous
scalarization of KN black holes in the EMS theory. We find that there is no
lower bound on the rotational parameter $a$, but its upper bound is given by
$M^2-Q^2 \ge a^2$. Also, we confirm that the high rotation enhances spontaneous
scalarization of KN black holes in the EMS theory.
