We propose an optically switchable ultra-broadband terahertz (THz) perfect absorber based on doped superlattice photonic-crystal silicon. The structure consists of a superlattice photonic crystal silicon slab capped by a SU-8 layer with no metal reflector. It achieves ultra-broadband perfect absorption due to the coupled cavity modes of superposed lattices with two different cavity radii and the enhanced diffraction assisted by the top SU-8 layer. Switchable absorption is realized by changing the carrier concentration of doped silicon controlled by pump beam. Simulation shows that proposed structure exhibits polarization-insensitive and wide-angle absorption with efficiency larger than 90% within 1.03 to 5 THz ultra-broadband, with modulation depth larger than 60% within bandwidth more than 1 THz. The all-dielectric structure uncommonly integrates optical switching, ultra-broadband absorption, polarization, and incident angle insensitivity, which may find potential applications in dynamic THz devices.