A method for detecting a response (grey level) of each probe zone on a test strip includes providing a test strip having a color pattern. The color pattern occurs in response to a solution contacting with the test strip and includes color lines arranged in sequence. Each color line represents a probe zone of the test strip. A whole image of the test strip is captured and is selected at least one scan line perpendicular to the color lines. A pixel position of the scan line having a minimum pixel value corresponding to a bottom edge of the test strip is set and the pixel position is used as a reference to identify respective pixel positions of the color lines on the scan line.