Soft magnetic tip was utilized to be the probe of spin-polarized scanning tunneling microscopy. It was demonstrated that the spin contrast can be reversed by in situ switching tip magnetization through varying tip-substrate distance for resolving perpendicular magnetic domain images. With this in situ magnetization direction switching of the soft magnetic tip, it is conceivable to separate magnetic from chemical and topographic contributions without applying external magnetic field. This provides an effective tool for the study of complex magnetic spin structures with various nonmagnetic impurities or compositions involved.