Abstract: In order to satisfy the industrial requirements of online thickness inspection for semi-transparent thin-film, a method based on optical transmission density was presented. The optical density could be calculated by output and input radiation captured by camera, which were grayscale value received with and without thin-film, respectively. Then relationship between density value and thickness was constructed by Beer-Lambert law. To reduce measurement error, it was necessary to calibrate the density by standard calibration sheet, and correct the density drift of calculated value. The system exploited thickness measurement had a simple configuration, and 100 mm * 75 mm range was measured each time. PEFT film was adopted to demonstrate the feasibility of the method. The analysis results showed that the average error of the densitometer was 5.7% with a standard deviation of 6.66%. It can be used to full-field online thickness measurement of thin film.