Considering the frequency dependence of waist size on the cavity modes, an analytical form of the ultrashort pulsed Gaussian beam is derived by using a complex temporal varible, which contains all the information about the spatial and temporal properties of the pulsed beam. The real part of the complex time gives the pulse traveling time, which is related to the wavefront. The imaginary part gives the information of spatial distribution and the crossed part gives the coupling between space and time on propagation of the pulse. Then the space-time characteristices of a transform limited Gaussian pulse and a Gaussian pulse with linear chirp are analyszed.