The characteristics of dynamic gratings greatly affect the linewidth and mode stability of ultra-narrow linewidth erbium-doped fiber (EDF) lasers. In this paper, we propose a novel method to measure the temporal evolution of the reflectance spectra of the dynamic gratings recorded in EDF based on the transient effect of the erbium ions by applying optical frequency modulation on the written light. The transient reflectance spectra of the linear configuration dynamic gratings are measured, and the influences of the written optical power and the terminal reflectivity on the response characteristics of the gratings are also studied. Experimental results show that the first order zero point frequency of the gratings formed in a 3-m-long erbium doped fiber is 30 MHz which accords with the value obtained from the steady state theory. The relative reflectivity change decreases with the increase of input optical power or terminal reflectivity. The measured maximal changes occur at low input power or terminal reflectivity. The grating building time also decreases with the increase of optical power, and it is less than 1 ms when input power is larger than 4 times the saturable power. This phenomenon can be explained by the process of two-wave mixing.