The chemical action, which is generally determined by acid or alkaline regulator in the slurry, plays an important role in fixed abrasive chemical mechanical polishing (CMP). Experiments were conducted to explore the influence of acid slurries on surface quality and material removal in fixed abrasive CMP of LBO (LiB3O5) crystal with soft and brittle, difficult-machining properties. Four organic acid regulators with pH 4.0 (acetic acid, lactic acid, citric acid, and oxalic acid) were screened by the polishing experiments. Further, pH value of the regulator was optimized to the most suitable chemical environment fit for polishing of LBO crystal using three indexes, i.e., material removal rate (MRR), surface topography, and surface roughness. The results indicated that citric acid leads to more robust polishing performance with surface roughness Sa (surface roughness average) 0.52 nm lower than the others. Additionally, MRR decreases with increasing pH value. Surface roughness initially decreases with increasing pH value, then it mutates to the maximum 9.37 nm after reach the minimum. The abrasive-free acid slurry with citric acid at pH 5.0 is effective for fixed abrasive CMP of LBO crystal, obtaining a MRR of 366 nm/min and fine surface quality with surface roughness Sa 0.32 nm with rather slight surface damage.