The mechanism of the second order susceptibility created in the fused silica by thermal/electric field poling is analyzed in detail in this paper. It shows that the second order susceptibility is established by both the orientations of dipoles and third order susceptibility activated by strong static electric field in the depletion region of the sample. Under the ordinary poling condition the former is the main factor, whereas if with the greater poling voltage the latter is the key factor. The expressions for calculating second order susceptibility are deduced, and the numerical calculation is performed. The simulation results show, under the ordinary situation, χ33(2) and χ33(2):χ31(2) are about 1 pm/V and 3, respectively, which are quite consistent with the experimental reports. The theory proves that second order susceptibility can be increased by improving the applied voltage and choosing the silica material with the greater concentration of Na and OH impurity.