  In this paper, we focus on the packing measure of self-similar sets. Let $K$
be a self-similar set whose Hausdorff dimension and packing dimension equal
$s$, we state that if $K$ satisfies the strong open set condition with an open
set $\mathcal{O}$, then $\mathcal{P}^s(K\cap B(x,r))\geq (2r)^s$ for each open
ball $B(x,r)\subset \mathcal{O}$ centered in $K$, where $\mathcal{P}^s$ denotes
the $s$-dimensional packing measure. We use this inequality to obtain some
precise density theorems for packing measure of self-similar sets, which can be
applied to compute the exact value of the $s$-dimensional packing measure
$\mathcal{P}^s(K)$ of $K$. Moreover, by using the above results, we show the
continuity of the packing measure function of the attractors on the space of
self-similar iterated function systems satisfying the strong separation
condition. This result gives a complete answer to a question posed by L. Olsen.
