Just made a brand-new folder and reinstall DB, using the latest version of Trinity(2.14.31) with the BarbarianCombat.cs that jubisman just provided.(Put it under X:\Demonbuddy 1.1.2914.450\Plugins\Trinity\Combat\Abilities)
After about 10 runs of nephalem rift, still got ~70% of the time with max fury without WOTB up.
But I noticed that if I cast ancient spear manually, DB will cast it for few times.
It's slightly better than before, but it's not fixed or maybe I just did this all wrong. :/
Thanks again.