  We derive the lower bound of uncertainty relations of two unitary operators
for a class of states based on the geometric-arithmetic inequality and
Cauchy-Schwarz inequality. Furthermore, we propose a set of uncertainty
relations for three unitary operators. Compared to the known bound introduced
in Phys.Rev.A.100,022116(2019), the unitary uncertainty relations bound with
our method is tighter, to a certain extent. Meanwhile, some examples are given
in the paper to illustrate our conclusions.
