Exchange bias effect has been widely employed for various magnetic devices. The experimentally reported magnitude of exchange bias field is often smaller than that predicted theoretically, which is considered to be due to the partly pinned spins of ferromagnetic layer by antiferromagnetic layer. However, mapping the distribution of pinned spins is challenging. In this work, we directly image the reverse domain nucleation and domain wall movement process in the exchange biased CoFeB/IrMn bilayers by Lorentz transmission electron microscopy. From the in-situ experiments, we obtain the distribution mapping of the pinning strength, showing that only 1/6 of the ferromagnetic layer at the interface is strongly pinned by the antiferromagnetic layer. Our results prove the existence of an inhomogeneous pinning effect in exchange bias systems.