  An algebraic group is called semi-reductive if it is a semi-direct product of
a reductive subgroup and the unipotent radical. Such a semi-reductive algebraic
group naturally arises and also plays a key role in the study of modular
representations of non-classical finite-dimensional simple Lie algebras in
positive characteristic, and some other cases. Let $G$ ba a connected
semi-reductive algebraic group over an algebraically closed field $\mathbb{F}$
and $\mathfrak{g}=Lie(G)$. It turns out that $G$ has many same properties as
reductive groups, such as the Bruhat decomposition. In this note, we obtain an
analogue of classical Chevalley restriction theorem for $\mathfrak{g}$, which
says that the $G$-invariant ring $\mathbb{F}[\mathfrak{g}]^G$ is a polynomial
ring if $\mathfrak{g}$ satisfies a certain "posivity" condition suited for lots
of cases we are interested in. As applications, we further investigate the
nilpotent cones and resolutions of singularities for semi-reductive Lie
algebras.
