The accuracy of modal test results is affected by uncertain influencing factors such as data acquisition， data processing， and parameter estimation. To study the uncertainty levels of modal parameters and their behaviors under different influencing factors in modal testing， a fast uncertainty quantification analysis method based on the state space model and Fisher information matrix （FIM） is adopted. The influence of multiple factors including data duration， sampling frequency， signal-to-noise ratio， and damping ratio of vibration signal on the uncertainty levels of modal parameters is discussed， and a practical way for optimizing the sensor layout scheme through uncertainty analysis is proposed. Results show that the uncertainty level of modal parameters decreases with the increase of data duration， sampling frequency， signal-to-noise ratio， and the number of sensors， and increases with the increase of damping ratio； for uniformly distributed structures， the majority of the sensors at the upper and middle region of the structure and only a few at the lower region is a preferred sensor layout scheme with small overall uncertainty of modal parameters.