The global amplitude of the westward propagating quasi-16-day wave (16DW) with wavenumber 1 (Q16W1), the strongest component of 16DW, is derived from European Centre for Medium-Range Weather Forecasts ERA-Interim reanalysis temperature data set from February 1979 to January 2018. The strong climatological mean amplitudes of the Q16W1 appear in winter in the upper stratosphere at high latitudes in both hemispheres, and the wave amplitude is stronger in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH). Multivariate linear regression is applied to calculate responses of the Q16W1 amplitude to QBO (quasi-biennial oscillation), ENSO (El Niño-Southern Oscillation), solar activity and the linear trend of the Q16W1 amplitude. The QBO signatures of the Q16W1 amplitude are mainly located in the stratosphere. In addition to the significant QBO response in the low latitude and low stratosphere, the largest QBO response occurs in the region with the strongest Q16W1 climatology amplitude. There no significant responses to ENSO and solar activity are observed. The linear trend of the monthly mean Q16W1 amplitude is generally positive, especially in the mid-high latitudes of the stratosphere. The trend is asymmetric about the equator and significantly stronger in the NH than in the SH. The trend shows obvious seasonal changes, that is, stronger in winter, weaker in spring and autumn. Further investigation suggests that the background and local instability trends contribute most of the increasing trend of the Q16W1 amplitude. In winter in both hemispheres, the weakening trend of eastward zonal wind provide more favourable background wind for Q16W1 upward propagation, in autumn and winter in the NH and in spring, autumn and winter in the SH, the increasing trend of local instability may enhance the wave excitation.