  We argue that the global signal of neutral hydrogen 21cm line can be a
powerful probe of primordial power spectrum on small scales. Since the
amplitude of small scale primordial fluctuations is important to determine the
early structure formation and the timing when the sources of Lyman ${\alpha}$
photons are produced, they in turn affect the neutral hydrogen 21cm line
signal. We show that the information of the position of the absorption trough
can severely constrain the small scale amplitude of primordial fluctuations
once astrophysical parameters relevant to the 21cm line signal are fixed. We
also discuss how the uncertainties of astrophysical parameters affect the
constraints.
