The multiscale analysis method is used to study the characteristics of temperature field under different sizes of normal and fire conditions based on the fire tests of Dalianlu Tunnel, Qingpu Tunnel and Baimang Xueshan Tunnel. With the help of the data of fiber bragg grating temperature system, the statistical descriptive analysis of temperature, temperature gradient and second derivative are studied. Meanwhile, the fluctuation characteristic of temperature field in tunnel in case of fire is discussed. According to the analysis, it is concluded that the temperature field is strongly fluctuating when the tunnel is on fire; and the standard deviation of the temperature and temperature gradient in the unit time and the dispersion coefficient can be used to characterize the fluctuation of the temperature field. The recommended values of standard deviation under normal conditions are given. Finally, the fire disaster can be cleared by identifying the temperature fluctuation characteristics of the ignition source and its adjacent domains.