Formaldehyde was determined by spectrophotometric method of 7.2 phenol reagent in GB/T 18204.2-2014 part 2: chemical pollutants. The effect of formaldehyde, phenolic reagent and colorant on the test results was verified by studying the reaction mechanism. The results show that when the formaldehyde content is too high(> 8.0 mg/mL) or the phenolic reagent concentration is high, the color reaction will not be carried out normally, there by affecting the experimental results.