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 u $ 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.
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