The propagation characteristics of Gaussian pulsed beams with first-order spatio-temporal coupling (FSTC) are investigated by means of Collins formula, and the analytical expression for the FSTC beam on the output plane is derived. The rotation angle of equiphase surface is defined to estimate the severity of phase couplings. The correlation coefficient and the rotation angle of equiphase surface are analyzed with the variation of propagation distance. The intensity distribution and phase distribution for different propagation distances are simulated. It is found that after passing a distance in the free space, the FSTC beam keeps the form, but the severity of spatio-temporal couplings, the intensity distribution and phase distribution of the FSTC beam vary with the propagation distance.