  Understanding the spin dephasing mechanism is of fundamental importance in
all potential applications of the spin qubit. Here we demonstrate a spin
dephasing mechanism in semiconductor quantum dot due to the $1/f$ charge noise.
The spin-charge interaction is mediated by the interplay between the spin-orbit
coupling and the asymmetrical quantum dot confining potential. The dephasing
rate is proportional to both the strength of the spin-orbit coupling and the
degree of the asymmetry of the confining potential. For parameters typical of
the InSb, InAs, and GaAs quantum dots with a moderate well-height $V_{0}=10$
meV, we find the spin dephasing times are ${\rm T}^{*}_{2}=7$ $\mu$s, $275$
$\mu$s, and $55$ ms, respectively. In particular, the spin dephasing can be
enhanced by lowering the well-height. When the well-height is as small as
$V_{0}=5$ meV, the spin depahsing times in the InSb, InAs, and GaAs quantum
dots are decreased to ${\rm T}^{*}_{2}=0.38$ $\mu$s, $18$ $\mu$s, and $9$ ms,
respectively.
