Abstract: In order to use fractional-order Fourier transform(FRFT) to effectively detect coefficient of chirp signal,characterization of linear chirped ultrashort pulse was studied based on FRFT. FRFT has characteristics of high energy aggregation in an optimal order. According to the characterization, the relationship between optimal order and chirped coefficient of ultrashort pulse was proved theoretically. Numerical simulation was done for Gaussian linear-chirped pulse. The relationship curve between the fractional order and the maximum of the result of FRFT was drawn. The results show that the theoretical formula is in agreement with numerical simulation. The conclusion can provide reference for discussing chirp coefficient of ultrashort pulse.