Fresnel biprism has been applied to the design of plasmonic meta-lenses recently. In order to promote these applications and understand the physics behind them, in this paper we investigate the focusing effect of Fresnel biprism from the perspective of information optics and geometrical optics. The expression for optical field intensity describing the focusing effect of Fresnel biprism is derived according to the relationship between the impulse response function and the optical field. Then the formula of the focal length is achieved. Furthermore, the Fresnel biprism focusing experiment is performed. Specially, the optical field intensity distribution is measured and the corresponding intensity along the axis is obtained. The results show that the focusing effect depends on the base angle, refractive index and base length of the biprism. There exists axial resonance effect in the axial intensity. The experimental results are in accordance with the theoretical results. These results could be valuable to the applications of Fresnel biprism in designing large depth of focus plasmonic meta-lenses.