The invention discloses a twisted steel for a 785 MPa-level high-strength prestressed structure and a preparation method of the twisted steel. The twisted steel comprises following constituents in percentage by weight: 0.50-0.55% of C, 0.80-1.10% of Si, 1.00-1.30% of Mn, 0.060-0.12% of V, 0.60-0.95% of Cr, 0.0015-0.0040% of B, 0.010-0.025% of S, 0.010-0.025% of P, and the balance of Fe and unavoidable impurities. The preparation method comprises steps of billet preparation, rolling and after treatment. In the preparation process, when vanadium is utilized to conduct microalloying and reinforcement, the alloying element B is appropriately added to improve the steel hardenability, and the addition of the microalloying reinforcing elements is lowered. As the controlled rolling technology is combined and the cooling technology is applied, the grain refinement of steel structure is obvious, the net-like ferritic structure at the core of the steel is lowered, the steel strength is increased, the hardness gradient change is slow, and the stability of the precision rolled twisted steel is improved. The processed steel is stable in technologic and mechanical performances, and good in prestress relaxation performance.