With high purity graphite becoming an important raw material in modern science, technology and industrial development, more and more fluoride-containing wastewater will be produced.The accurate monitor plays an important role in the control of the industry's environmental impact. It is found that fluorine ion concentration measured by fluoride ion selective electrode increased with the increasing of dilution ratio in graphite refining wastewater measurement. Reasons like dissolution of suspended calcium fluoride micro-crystal, the interference of impurity ions and the effect of ionic strength were analyzed through water quality analysis and theoretical calculation. Ion chromatography was used to check the result of experiments. Results show that the release of fluorine ion with complexing state from masking agent of TISAB is the main reason of what was found in the experiment. The fluorine ion concentration of graphite refining wastewater is about 0 to 30 mg/L.The water needs to be diluted 100 times when the fluoride ion concentration is measured by the national standard method.