A test apparatus for measuring the temperature of a reactor using a thermochromic pigment, and a method for controlling the test apparatus are disclosed, based on a technology for irradiating light of different wavelengths on a thermochromic pigment accommodated in a reactor and estimating temperature of the reactor using a difference between absorbance values corresponding to the light of the different wavelengths. The test apparatus includes at least one light emitter configured to irradiate light of different wavelengths onto a chamber included in the reactor, a light receiver configured to receive the light that propagates through the chamber, and a controller configured to measure absorbance values of the thermochromic pigment in correspondence to the different wavelengths of the light, to calculate a difference between the measured absorbance values, and to determine a temperature of the reactor in correspondence to the calculated difference between the absorbance values.