Single point and scan exposure experiments on ultraviolet laser inscribed type I fiber Bragg grating by focused infrared femtosecond laser are reported. The effect of femtosecond laser pulses on the grating transmission spectrum is studied especially when the pulse energy density is far lower than the fiber damage threshold. Experimental results show that the grating spectrum during irradiation shows a red shift, and the spectral structure is no longer a single transmission peak but more irregular transmission peaks. However, after stopping exposure, the resonant wavelength shows blue shift phenomemon and the transmissivity increases, which tend to be saturated with the time increasing. The grating exposure region is considered to be in a non-uniform temperature field, and the laser-induced refractive index change overlaps the original grating refractive index modulation distribution. The spectrum change during exposure and after a certain time ′s exposure is theoretically simulated and the simulation results fit well with the experimental ones.