This paper takes the platform of Donghua gate in the Forbidden City as monitoring object.Based on the Brillouin optical time domain reflectometer（BOTDR）

the deformation and leakage of north and west wall of platform as well as the deformation of middle doorway have been monitored.The monitoring results show that the measured strain values of major sensing fiber segments are distributed in the measurement error range of ±100με;Only partial lateral sensing fiber segments of the north wall present tensile strain

but the strain values are in the range from 150μεto 250με

which indicates that a slight drum deformation has occurred in the upper local area of west wall.It can be inferred that the deformation is caused by the curing of the wall surface plaster and the thermal expansion and contraction are caused by temperature changes.The abnormal region of temperature change was not be monitored

which indicated that the leakage of platform didn′t happen.Therefore

we can judge the overall structure of the platform of Donghua gate is stable at present.