Abstract: A 1.5 m hybrid undulator has been constructed for mid-infrared free electron laser (FEL) experiment at China Institute of Atomic Energy (CIAE). Its rms peak magnetic field error is 0.5%, magnetic field steering error is lmT·cm, electron trajectory deviation is 0.03 mm. The design of undulator, optimized sorting of magnets in undulator, magnetic field measurement technique and magnetic field adjustment method are described.