Volatile fatty acids (VFAs) are high-quality carbon sources for biological nitrogen and phosphorus removal. In order to further increase the yield of VFAs during anaerobic fermentation of waste activated sludge (WAS), and in view of the actual situation of the development of vinegar industry in Shanxi Province, we considered the co-fermentation of vinegar residue (VR) with WAS to explore the changes of VFAs production. What’s more, the effect of different feedstock proportion on VFAs production was studied. In this paper, the acidification performance was compared by setting up three experimental group: the WAS alone fermentation as Control, and the co-fermentation of WAS and VR as experimental group (experimental group 1: VSSWAS:VSSVR=2:1; experimental group 2: VSSWAS:VSSVR=1:2). Experimental results showed that: co-fermentation system can significantly improve the VFAs production. And the maximum VFAs concentration of group 1 and 2 were 4856 mg COD/L and 8595 mg COD/L, which was 1.3-fold and 2.4-fold of that in Control (3675 mg COD/L), respectively. Furthermore, the addition of vinegar residue can promote the production of small molecular weight acid (acetic acid). Adding vinegar residue, not only brings the soluble carbohydrate, but also can adjust the C/N ratio during anaerobic fermentation of WAS, which can improve the soluble organics’ hydrolysis and provide basic substances for VFAs production.