  We propose to synthesize tunable periodic gauge fields via Floquet
engineering cold alkaline-earth atoms in one-dimensional optical lattice. The
artificial magnetic flux is designed to emerge during the combined process of
Floquet photon assisted tunneling and internal state transitions. By varying
initial phases of driving protocol, our proposal presents the ability to
smoothly tune the periodic flux. Moreover, we demonstrate that the effective
two-leg flux ladder model can simulate one typical real topological insulator,
which is described by the first Stiefel Whitney class and protected by the $PT$
symmetry. Benefiting from the long lifetime of excited states of alkaline-earth
atoms, our work opens new possibilities for exploiting the physics related to
gauge fields, such as topological phases, in the current cold atom platform.
