Abstract: Periodic fluctuation of agrometeorological thermal factors is investigated by using methods of spectrum analysis and periodogram. Analysis on the medium to short periods of active and effective accumulated daily mean temperature above 100C and it's duration in Shanghai, Tsingtao and Tientsin show that period lengths of about 7-8 and 2-3 years are prominent, especially that of 8 years. The periodic fluctuation of effective accumulated temperature is more stable.