  A relatively complete picture on the steady states of the following
Schr$\ddot{o}$dinger-Poisson-Slater (SPS) system \[ \begin{cases} -\Delta
Q+Q=VQ-C_{S}Q^{2}, & Q>0\text{ in }\mathbb{R}^{3}\\ Q(x)\to0, & \mbox{as
}x\to\infty,\\ -\Delta V=Q^{2}, & \text{in }\mathbb{R}^{3}\\ V(x)\to0 &
\mbox{as }x\to\infty. \end{cases} \]
  is given in this paper: existence, uniqueness, regularity and asymptotic
behavior at infinity, where $C_{S}>0$ is a constant. To the author's knowledge,
this is the first uniqueness result on SPS system.
