Abstract: Structural modal parameters (frequencies, damping ratios and mode shapes, etc.) play a critical role in structural health monitoring and dynamic test. Knowing their identification uncertainty can significantly enhance the accuracy and robustness of structural damage detection and condition assessment. Focusing on ‘understanding’ rather than ‘computing’ the identification uncertainty, asymptotic uncertainty gives the analytical forms of the coefficient of variation (c.o.v.) of identified modal parameters regardless of the particular dataset and identification algorithm. This paper introduces the asymptotic identification uncertainty of modal parameters in forced vibration test with known single input. Based on the long data and small damping assumptions, asymptotic expressions for the posterior c.o.v. of structural frequency, damping ratio and mode shape are given. A series of field tests are carried out for verification. The asymptotic identification uncertainty can be used to guide the practice of dynamic testing, to resolve the blindness of the dynamic test and the passiveness of the modal identification.