The invention relates to cold continuous rolling bake hardening steel with yielding strength of 300 MPa grade and a preparation method thereof, and solves the problem of weak product forming performance due to the design of existing bake hardening steel with the yielding strength of 300 MPa grade by adopting low-carbon components. The steel comprises the following chemical components in percentage by weight: 0.0018-0.0030% of C, 0.20-0.30% of Si, 0.10-0.12% of P, 0.60-0.70% of Mn, 0.02-0.06% of Als, 0.006-0.012% of Nb, limiting elements: S below 0.015%, N below 0.004%, and the balance is Fe. A slab satisfying the requirements of the chemical components is obtained by the smelting and the continuous casting; and the finished steel is obtained by performing the heating, the hot rolling, the winding, the acid washing, the cold rolling, the continuous annealing and the leveling for the slab. The steel has the advantages of simple process, excellent forming performance, high yielding strength, low raw material cost and low production cost.