During process of electric power tower cross arm long-term operation,the stress of insulation cross arm change complex.Real-time monitoring on cross arm stress is an important factor that ensure long-term security and stability of tower cross arm.Put forward a kind of fiber Bragg grating(FBG)strain sensor which can transform stress variation beared by cross arm into wavelength shift of FBG,and applied in substation of power tower.According to the climatic feature of Yan Jin transformer substation,install four strain sensors in the center of the tower truss material up and down surface.Aiming at the climatic conditions on February,2011,strain monitoring range is -58.05 *10~(-6)~ -242.52*10~(-6); snow,rain weather occur at 10 o'clock,on January 12,2011,the strain monitoring range is -93.71*10~(-6)~-3.46*10~(-6) and four strain sensors simultaneously fluctuations at the same time.The experiment indicates that in normal circumstances,cable dance,snow,rain and other weather conditions are the important factors which cause cross arm deformation of tower.