Disclosed are a multi-mode thermal imaging device and an operation method thereof. According to an embodiment of the present invention, in a first mode, a first sample is scanned with an optical signal from a light source, signals reflected from the first sample by the scanning are detected separately for each wavelength, a reflectance change spectrum according to the wavelength is derived on the basis of the signals detected separately for each wavelength, a wavelength is selected on the basis of the derived reflectance change spectrum, and a thermal image of the first sample is obtained, through a filter, by detecting an optical signal limited to the selected wavelength from among the signals reflected from the first sample. In a second mode, thermal radiation of a second sample is detected to obtain a thermal image of the second sample.