The absorption detection of optical coatings is studied in theory and application. A novel method of multichannel measurement based on image lock-in is presented. The comparison between the calculated measuring results of point by point method and multichannel method is made, which proves the feasibility of the multichannel method for large scaled coatings measurement. Through calculation, it is known that the measuring speed of image method can be increased more than hundreds of times compared with that of the past. This technique avoids the weakness of long time-consuming and low efficiency in traditional method and expands the application of photothermal technique in large scale optical coatings.