A lot of methods were proposed to design observer for
　　reconstructing the unknown input when the so-called
　　observer
　　matching condition was met. The problem of designing
　　observer and reconstructing the unknown input was
　　considered when the
　　system does not meet the condition. The idea of
　　generating auxiliary outputs was adopted to meet the
　　matching condition. Then the
　　high-gain observer was used to obtain the estimates
　　of auxiliary outputs by which a reduced-order
　　observer is designed to estimate
　　the unknown input. The simulation results of a real
　　model show that the proposed method is effective.