  We have analyzed the total intensity, spectral index, linear polarization,
and RM distributions at pc scale for the quasar 1604+159. The source was
observed in 2002 and 2020 with the VLBA. Combining the MOJAVE results, we
studied the evolution of the magnetic field. We detected a core-jet structure.
The jet extends to a distance of ~25 mas. The jet shape varies slightly with
time. We divided the source structure into the central region and the jet
region. In the jet region, we find the polarized emission varies with time. The
flatter spectral index values and EVPA direction indicate the possible
existence of shocks, contributing to the variation. In the central region, the
derived core shift index k_r values indicate that the core in 2002 is close to
the equipartition case while deviating from it in 2020. The measured magnetic
field strength in 2020 is two orders of magnitude lower than that in 2002. We
detected transverse RM gradients, evidence of a helical magnetic field, in the
core. At 15 GHz, in the place close to the jet base, the polarization direction
changes significantly with time from perpendicular to parallel to the jet
direction. The evolution of RM and magnetic field structure are potential
reasons for the observed polarization change. The core |RM| in 2020 increases
with frequency following a power law with index a = 2.7, suggesting a fast
electron density fall-off in the medium with distance from the jet base.
