This paper theoretically analyzed the influence of the guide rail irregularity on calibration of very-broadband seismometers under long stroke motions on shake table, and then quantitatively analyzed the oblique effect on calibration of seismometers on shake table due to the guide rail irregularity at 0.1 Hz and lower frequencies. The results indicate that oblique effect has a larger impact on sensitivity of the horizontal component than that of the vertical component. Higher-precision level meter was used to get the motion trace of the horizontal shake table under long stroke. Based on the influence mechanism of the guide rail irregularity and the data obtained from the level meter, a dynamic compensation system was designed with the software language of Labview. Test results suggest that the calibration accuracy is significantly improved at lower frequencies. Therefore, the dynamic compensating method is effective for very-broadband seismometer testing on shake table.