The invention discloses a trapezoidal precast slab with an auxiliary block and a construction method thereof. The auxiliary block is arranged below the trapezoidal precast slab and is of a reinforcedconcrete structure, an anchoring rib is arranged outside the auxiliary block, the top of the auxiliary block supports the precast slab, a vibration isolation element is arranged between the auxiliaryblock and the precast slab, the auxiliary block and a base are poured into a whole, the top of the auxiliary block is higher than the top of the base, so that the space between cast-in-place concreteand the precast slab is increased compared with that in a prior method, isolating materials and wrapping material in the prior method can be saved, the precast slab is prevented from sticking with thefoundation of the cast-in-place concrete, in addition, increased space facilitates vibration and plastering operations of the concrete, which avoids hanging between the vibration isolation element and the foundation of the concrete, and finally, reinforcing steel bars with protrusions, caused by irregular binding at a track foundation, can be prevented from bearing the vibration isolation elementor the precast slab; the trapezoidal precast slab is also provided with a spiral column height adjusting structure capable of adjusting the height of the trapezoidal precast slab.