A new two-beam-contracted radial shearing interference optical system is developed to study on the large field of gas temperature. Methods of fast Fourier transform (FFT) and phase unwrap were used to demodulate the phase of the interferogram obtained from experiment, distortion of wavefront disturbed by temperature fluctuation which equals to phase shift induced by refractive index change was calculated, and a way of axisymmetric temperature field reconstruction was put into reshowing a layer of the gas temperature distribution. By comparing, the difference is equivalent to that of the mini-thermocouple. Experimental result shows that the optical system is reasonably sensitive and precise. It is suitable for measuring the phase object of which the lateral size is the larger and the longitudinal size is the shorter, when the difference of the temperature is little compared with the environment.