The influence of reflow soldering, one of the instantaneous thermal shocks, on the frequency stability of quartz crystal resonators is analyzed and studied. Based on the existing research experience and the frequency recoverability after reflow soldering, it is deduced that the film stress is the main influence factor of frequency stability after reflow soldering.Through experiments,it is found that the frequency stability of resonators after reflow soldering can be effectively improved when the sputtering power of the thin film is reduced to 200 W or the annealing temperature of the thin film is increased to 350 ℃, and the frequency deviation after reflow soldering decreases by about (4~5)×10-6. The micro Raman spectrometer is used to analyze quartz wafers before and after reflow soldering, and it is found that the Raman characteristic peak of quartz wafers shift obviously toward the high frequency after reflow soldering. It is inferred that the compressive stress of the quartz wafers increase by about 8 MPa because of the increase of the tensile stress of thin film caused by its thermal expansion.With the increase of the cooling time, the thin films shrink and the pressure stress of quartz wafers decreases and the frequency gradually recovers. The results show that the change of film stress is the mechanism of reflow soldering on the frequency stability of resonators.