A new infrared thermal image method to measure the starting duration of YBCO high-temperature super-conductor (HTS) film chemical etching was proposed. The temperature variation of liquid film on YBCO surface was induced by absorption or release of chemical heat during etching of YBCO film, and the start time of reaction can be tested by real-time collection of the infrared thermal image in the course of liquid film temperature variation. Theoretical analysis and experimental results show that linear liquid film with 2 mm width is adtopted to monitor because all the information of temperature and space change is possessed, and the center of liquid film can be set as an observed character point. The more accurate experimental data can be obtained because of the high sensitivity of temperature change of extraordinary thin linear film made by sliding droplet. The influence of film weight on start time can be avoided with upright YBCO film (LaAlO3 substrate). The start time of reaction between YBCO film and H3PO4H2O( volume ratio 1:300)liquid is about 0.3～0.4 s, which is obtained by the grey-scale variation of a certain cross section of linear liquid film.