A rather whole mathematical model of multilateral horizontal pinnate well for CBM recovery is developed. The coalbed is considered as dual medium and the factors including heterogeneity and anisotropy of stratum, sensitivity of permeability to pressure, friction and acceleration loss in well bore and startup pressure derivative in low permeability coalbed are considered in this model. The mechanism of multilateral horizontal pinnate well for CBM recovery is analyzed by using of simulation technique. It is pointed that whole pinnate well bore spreading widely and evenly in coalbed makes stratum pressure dropped evenly and fast, which increases the chance of desorption and diffusion in area controlled. The above is the basic reason that horizontal pinnate well can increase production. The existence of startup pressure gradient can make depressurization effect of horizontal pinnate well get worse, which promotes gas production reducing.