Average-consensus filter problem is investigated for a mixed-order multi-agent system, which consists of first-order and second-order agents, and the proportional-integral consensus filter algorithms are proposed for the agents with different constant inputs. Based on generalized Nyquist stability criterion, sufficient convergence conditions are obtained for the multiagent system under a fixed, symmetric and connected topology to achieve the asymptotic consensus seeking of average value of agents' constant inputs. Besides, the delay-dependent convergence conditions are obtained for the multi-agent systems with identical communication delay. Numerical simulations are presented to illustrate the correctness of our results.