Abstract: When a severe accident happens, the dynamic change of the reactor core is particularly important for tracking accident progression and determining the emergency plan. The cosmic ray muon imaging technique is sensitive to high atomic number materials with high penetration, which has been demonstrated to be applicable for monitoring of nuclear materials. The reconstruction algorithm is a key point of this technique, and the detective object is usually accompanied by noises with the similar gray value for its high atomic number. In order to improve reconstruction quality for real-time monitoring the change of reactor core during a severe accident for emergency staffs, a reconstruction algorithm using POCA accompanied by Butterworth filter was proposed in this paper. The primary simulation result demonstrates that this algorithm which can obviously reduce the noise is effective.