The invention relates to an automobile beam steel plate with tensile strength of 750MPa grade and heavy gauge, and a preparation method of the automobile beam steel plate. The steel plate comprises, by mass, 0.03 to 0.12% of C, 0.05 to 0.60% of Si, 0.2 to 1.0% of Cr, 1.0 to 2.5% of Mn, not greater than 0.015% of S, not greater than 0.025% of P, 0.015 to 0.045% of Als, 0.07 to 0.15% of Ti, 0.02 to0.08% of Nb, and the balance being Fe and impurities that are unavoidable during smelting. During preparation, a steel billet adopting the above chemical composition proportion is subjected to reasonable process design, so as to prevent the pickling preparation process from replacing the original process, reduce the thickness of the scale, and obtain the scale complex structure of Fe3O4, FeO and eutectoid Fe3O4 coexisting with alpha-Fe, so that the surface quality of the steel plate is significantly improved, and the requirements of the use of pickling-free direct stamping or roll forming andeliminate the pickling process can be met, production efficiency is improved, environmental pollution is reduced and production cost is reduced.