Abstract: In view of the present status of numerical simulation of the wave tanke,three important problems of simulating long-crested irregular waves are presented in this paper: wave spectrum expansion,discussion on waves’ attenuation mechanism and an evaluation criteria for numerical simulation accuracy of the long-crested irregalar waves.The Wen Shengchang spectrum is taken as a target.The long-crested irregular waves are reproduced by using the viscous numerical wave tank simulation technique.The main causes of waves’ attenuation in numerical simulation are analyzed,and the factors affecting the attenuation are got in the study.The relative errors of numerical simulation are calculated by using the standard of CB/T 3675-1996.The evaluation criteria for simulating long-crested irregular waves are thus proposed.