Embodiments of methods for forming a staircase structure of a three-dimensional (3D) memory device are disclosed. In an example, a first plurality of stairs of the staircase structure are formed based on a first photoresist mask. Each of the first plurality of stairs includes a number of divisions at different depths. After forming the first plurality of stairs, a second plurality of stairs of the staircase structure are formed based on a second photoresist mask. Each of the second plurality of stairs includes the number of divisions. The staircase structure tilts downward and away from a memory array structure of the 3D memory device from the first plurality of stairs to the second plurality of stairs.