Although a superhydrophobic surface has great potential to delay frosting, it tends to become frosted under humid conditions and needs to defrost periodically. So far, the exact mechanism of defrosting still remains unclear. Here, we investigate the frost self-removal mechanism during defrosting on vertical superhydrophobic surfaces. Two self-removal modes are observed: peeling off and jumping off. When the frost thickness is larger than a threshold value, peeling off mode occurs; otherwise, jumping off mode takes place. Compared with the peeling off mode, the jumping off mode is less effective in self-removing frost as jumping is limited by energy transformation. A theoretical model based on frost melting-water permeation mechanism is proposed to determine the threshold value of frost thickness. According to this model, dependent on the frost porosity and the surface temperature (or heat flux). For the threshold value of the frost thickness is our particular experiments, the threshold value of the frost thickness predicted by the proposed model agrees well with our experimental results. Our work may advance the defrosting applications of superhydrophobic surfaces in related engineering fields.