  The sea quark contribution to the nucleon spin is an important piece for a
complete understanding of the nucleon spin structure. The production of W
bosons in longitudinally polarized $p + p$ collisions at RHIC provides an
unique probe for the sea quark polarization, through the parity-violating
single-spin asymmetry, $A_L$. At the STAR experiment, W bosons through the
leptonic decay channel $W \to e \nu $ can be effectively determined with the
Electromagnetic Calorimeters and Time Projection Chamber at mid-rapidity. The
previous STAR measurements of $A_L$ for W boson production from datasets taken
in 2011 and 2012, have provided significant constraints on the helicity
distribution functions of $\bar u$ and $\bar d$ quarks. In 2013 the STAR
experiment collected $p+p$ data with an integrated luminosity of about 300
pb$^{-1}$ at $\sqrt s$ = 510 GeV with an average beam polarization of about
$56\%$, which is more than three times larger than the total integrated
luminosity of previous years. The preliminary results of W-boson $A_L$ from
2013 data sample will be presented.
