The invention provides a preparation method of B-free hot formed steel and belongs to the technical field of high-strength steel for automobiles. The chemical ingredients of the steel comprise, by mass, 0.25%-0.5% of C, 0.7%-1.2% of Si, 0.5%-1.3% of Mn, 0.01%-0.08% of Alt, 1.0%-3.0% of Cr, 0.015% or smaller of P, 0.008% or smaller of S, 0.02%-0.09% of Ti, 0.05%-0.25% of Mo, 0.02%-0.09% of Nb and the balance Fe and inevitable impurity elements. The hot formed steel is obtained through smelting, hot rolling, cold rolling and tunneling, and the hot formed steel is subjected to austenitizing quenching. In the preparation process, the hot-rolled structure is required to comprise, by volume, 85%-95% of pearlite and 15% or smaller of ferrite, the quenched structure is martensite completely, and the tensile strength of the quenched steel reaches up to 1900-2100 MPa. No strong oxide or nitride forming element needs to be specially added into steel, the boron brittleness phenomenon can be effectively avoided in the preparation process, the mechanical property of the prepared hot formed steel reaches the international leading level, and the purpose of automobile weigh reduction can be effectively achieved.