In order to qualitatively test linearity of ultraviolet-visible (UV-VIS) spectrophotometer, we test a food-dye absorbance at a certain wavelength using a UV-VIS spectrophotometer based on the water quality standard liquid calibration method, and then use a basic mathematical method to calculate volume concentration at each test point. We get a linear relationship of volume concentration and absorbance. Experiments show that this method is easy and convenient to measure the linearity of the instrument and the calculation method is accurate and easy to understand.