An inertial sensing input apparatus is disclosed, which includes: an accelerometer module, capable of detecting accelerations with respect to a Cartesian coordinate system of X-, Y-, and Z-axes; and a gyroscope, used for detecting a rotation measured with respect to the Z-axis. By the aforesaid input apparatus, an input method can be provided which comprises steps of: (a) defining base signals with respect to each of such sensing elements; (b) detecting and determining whether Z-axis accelerations are varying; (c) enabling the input apparatus to enter a surface (2D) operating mode while no acceleration varying is detected along the Z axis; (d) enabling the input apparatus to enter a space (3D) operating mode while acceleration varying is detected along the Z axis.