Spatial broadening of ultrashort laser pulses with a certain spectral width is studied after passing through a pair of parallel gratings. Firstly，the formulas for calculating the spatial broadening of ultrashort laser pulses after passing through a pair of gratings are derived by using the conventional grating equation and geometric optics knowledge. The experimental results also show that with an increase of the distance between grating pair，the transverse axis length of the pulsed spot increases and fits very well with the theoretical value. Finally，the spatial spectral distribution of the broadened pulsed spot is measured point by point by a fiber spectrometer，and the result verifies appearance of the spatial chirp characteristics.