An online synchronization measurement method for loss and refractive index growth of fiber Bragg grating (FBG) is demonstrated. The dynamic evolution of loss in FBG during the inscription process is studied in detail. The results show that the loss coefficient α of FBG linearly increases with its coupling coefficient κ. In order to optimize the loss performance of FBG, we propose the loss-coupling slope coefficient α/κ to evaluate the loss characteristics of FBG. The results show that we can make the FBG loss-coupling slope coefficient larger by increasing the distance between the optical fiber and the phase mask or using the phase mask with higher diffraction efficiency of 0 order diffracted light, which may be caused by the increase of the background refractive index which is independent of coupling coefficient. And loss-coupling slope coefficient α/κ of the FBG in D2-loading fiber is 50% lower than at 1550 nm, compared with it in H2-loading fiber.