The invention discloses a method for selecting phosphate by waste acid in a sulfate process titanium dioxide production process. The waste acid generated in the sulfate process titanium dioxide production process is used for replacing a phosphate ore dressing pH regulating agent; phosphate ore is subjected to wet grinding until ore pulp capable of reaching the substance monomer dissociation fineness is obtained; the waste acid generated in the sulfate process titanium dioxide production process is added into the ore pulp; the materials are fully stirred until the pH value of the ore pulp is regulated to be 4 to 5; the regulated ore pulp enters a flotation machine in a normal way; meanwhile, a convention collecting agent is added; through air inflation and bubble scrapping; phosphate concentrate is obtained. The phosphate concentrate quality can be about improved by 0.5 to 1.0 percent; the recovery rate is improved by about 0.5 percent. After the waste acid is used, during phosphate flotation, the cost of the pH value regulating agent can be reduced by 4.8yuan/ton crude ore. The cost for treating one ton of titania waste acid is 100 yuan, so that 300 thousand tons of floating device need to consume 3600 tons of the regulating agent of 98-percent sulfuric acid, and 18 thousand tons of titanium dioxide waste acid can be consumed.