  We propose a scheme to overcome the great challenge of polarization loss in
spin-polarized ion acceleration. When a petawatt laser pulse penetrates through
a compound plasma target consisting of a double layer slab and prepolarized
hydrogen halide gas, a strong forward moving quasistatic longitudinal electric
field is constructed by the self-generated laser-driven plasma. This field with
a varying drift velocity efficiently boosts the prepolarized protons via a
two-stage coherent acceleration process. Its merit is not only achieving a
highly energetic beam but also eliminating the undesired polarization loss of
the accelerated protons. We study the proton dynamics via Hamiltonian analyses,
specifically deriving the threshold of triggering the two-stage coherent
acceleration. To confirm the theoretical predictions, we perform
three-dimensional PIC simulations, where unprecedented proton beams with energy
approximating half GeV and polarization ratio $\sim$ 94\% are obtained.
