In this paper, we use the type Ia supernova data to constrain the model of holographic dark energy. For d=1, the best fit result is Ω^0_m = 0.25, the equation of the state of the holographic dark energy w^0_=̣ -0.91 and the transition between the decelerating expansion and the accelerating expansion happened when the cosmological red-shift was z_T=0.72. If we set d as a free parameter, the best fit results are d=0.21, Ω_m^0=0.46, w_^̣0=-2.67, which sounds like a phantom today, and the transition redshift is z_T=0.28.