The invention relates to a heat treatment method of improving the depth of a hardened layer of a steel rail, belongs to the field of production processes of steel rail materials and aims to solve the technical problem of providing the heat treatment method of improving the depth of the hardened layer of the steel rail. The heat treatment method comprises the steps of naturally cooling the steel rail after finish rolling until the center temperature of a rail head tread is 660-730 DEG C, carrying out accelerated cooling until the center temperature of a rail head tread is 500-550 DEG C at the cooling speed of 1.5-3.5 DEG C/s, increasing the cooling speed to 1.0-2.0 DEG C/s, stopping accelerated cooling when the center temperature of a rail head tread is lowered below 450 DEG C, and carrying out air cooling to room temperature. With the adoption of the heat treatment method, the hardened layer with the thickness of 25mm at the rail head part can be obtained, the position which is 25mm below the surface layer of a rail head has the hardness value equal to that of the surface layer of the rail head, and in addition, the whole section of the rail head is in a pearlite structure, so that the improvement of the good service performance of the steel rail after being continuously worn due to the contact of a wheel and the steel rail is facilitated.