Three-dimensional (3D) image reconstruction of an Hα flare near the solar disk center, is done by a series of line-center and off-line-center filtering observation, according to the physical meaning of contribution function based on the line formation theory. The contribution function represents the contribution from stellar atmospheric layers to the emergent specific intensity and describes the depth distribution of escape photons. In this paper, an Hα flare of 26 March 2001 is used to illustrate the 3D line-of-sight reconstruction and its development.