In the present study, a new preconcentration method was developed to quantify the free formaldehyde (FA) in hair cosmetics by combining ultrasound-assisted cloud-point extraction with spectrophotometry. The method is based on the ion association of FA with cationic phenothiazine group dye, toluidine blue in presence of sulfite at pH 5.0, and then extraction of the formed complex into the micellar phase of nonionic surfactant, Triton X-114. The analyte extracted into the micellar phase was diluted with ethanol, and then detected at 630 nm by spectrophotometer. Under optimal conditions, a good linear relationship was obtained in the range of 2-120 mu g/L with a detection limit of 0.38 mu g/L. By preconcentration of 15 mL sample, the calibration sensitivity increased by 74-fold. The intra- and inter-day precisions expressed as RSDs are lower than 4.7 and 5.2%, respectively. Compared with other techniques, the study shown here provides a simple, fast, accurate, and reliable method for the analysis of FA in cosmetic products. The contents of FA of the samples are in the range of 3.4-25.1 mg/kg with an RSD lower than 4.6%. The results were statistically in good agreement with those obtained by an independent comparison method. Highlights: A new method was developed for the preconcentration of free formaldehyde from hair cosmetics. The method is simple, fast, precise, selective, and sensitive to detect trace formaldehyde. The precision is lower than 5.2% with recovery higher than 95% after spiking. The method was validated by comparing the results with those of independent method. The formaldehyde levels of hair cosmetics were reliably monitored and determined.