考虑到腔模光腰尺寸的频率依赖性，得到了一短脉冲高斯光束在自由空间传播的时域解析形式，发现其可由一复时间变量来描述。复时间变量的实部给出了脉冲的传输时间，其与波前曲率有关，虚部给出了空间量值，两者的交叉项称为时空耦合。并对傅里叶变换极限脉冲和啁啾脉冲的时空形式作了分析 Considering the frequency dependence of cavity waist size, a time-domain analytical form of the propagation of a short pulsed Gaussian beam in free space is obtained and it can be described by a complex time-varying variable. The real part of the complex time variable gives the transmission time of the pulse, which is related to the wavefront curvature, the imaginary part gives the magnitude of the space, and the cross term of the two is termed spatio-temporal coupling. The space-time forms of Fourier transform limit pulse and chirp pulse are analyzed