In this paper, Fourier transform spectroscopy method has been reported via using the Michelson-Morley interferometer with a 0.4nm spectral resolution and the spectral range from 700nm to 1800nm. Also, in order to calculate the absorption spectrum of materials, the spectrophotometer has been fabricated with accuracy 1nm and the spectral range from 1500nm to 4500nm. The absorption spectrum of Polystyrene has been measured by using this spectrophotometer. Optical signal, after the detection with PbSe and Ge detector and amplification 10⁵ times, is sent to computer by 14-bit analog to digital converter. These data are analyzed to take Fast Fourier transformm (FFT) then results as signal spectral curve is plotted in Labview environment. It is detected signal to noise ratio of this system is better than 5000.