On the basis of previous research work，this paper proposes a foreshock or generalized foreshock recognition spectrum deviation method． As a judge index，the spectrum of moderate earthquake events is wide for ordinary seismicity，while moderate earthquake with the nature of foreshocks or generalized foreshocks has the characteristics of a narrow frequency band，and it is deviated to the low frequency． The root cause may be related to metastable extension in the source or related area before main shock rupture or to regional fragmentation damage，crack nucleation process． An earthquake with MS7． 1 occurred on April 14，2010，in Yushu，Qinghai Province with a foreshock event with MS4． 7 and a magnitude 7． 3 earthquake happened on February 12， 2014，in Yutian，Xinjiang with a foreshock event with MS5． 3． The calculation results of these two foreshocks show that the spectrum of foreshocks all had deviations，and they are quite different from non-foreshocks seismic spectrum of equal size． Therefore，this result can verify the validity of the spectrum deviation method．