Let $N^{n+p}_q(c)$ be an $(n+p)$-dimensional connected indefinite space form of index $q$ $(1\leq q\leq p)$ and of constant curvature $c$. Denote by $\varphi:M\to N^{n+p}_q(c)$ the $n$-dimensional spacelike submanifold in $N^{n+p}_q(c)$, $\varphi:M\to N^{n+p}_q(c)$ is called a Willmore spacelike submanifold in $N^{n+p}_q(c)$ if it is a critical submanifold to the Willmore functional $W(\varphi)=\int_M\rho^ndv=\int_M(S-nH^2)^{\frac{n}{2}}dv$, where $S$ and $H$ denote the norm square of the second fundamental form and the mean curvature of $M$ and $\rho^2=S-nH^2$. If $q=p$, in \cite{s14}, we proved some integral inequalities of Simons' type and rigidity theorems for $n$-dimensional Willmore spacelike submanifolds in a Lorentzian space form $N^{n+p}_p(c)$. In this paper, we continue to study this topic and prove some integral inequalities of Simons' type and rigidity theorems for $n$-dimensional Willmore spacelike submanifolds in an indefinite space form $N^{n+p}_q(c)$ ($1\leq q<p$).