The method of calculating the posterior cramer-rao lower bound(PCRLB) for the extent target is proposed to address the performance evaluation for estimation of an extended target with clutters and missed detections. Supposing that the number of measurements from the extend target and false alarms have a Poisson distribution respectively, the method using information reduction factor is proposed, which can calculate the PCRLB with clutters and missed detections. Simulation results show that the proposed PCRLB can effectively show the performance limits of the extended target estimation algorithms. The higher the probability of detection is, the smaller the PCRLB will become. The greater the PCRLB is the bigger the clutter density will be.