Under the illumination of 800 nm femtosecond laser pulse, two-photon absorption phenomenon can occur in the photosensitive glass As2S3.　 When a chirped femtosecond laser pulse meets with another femtosecond laser pulse in an As2S3 fibre, the interference pattern can be recorded permanently as a chirped grating. In this paper, the relation between the structure of the chirped fibre grating and the parameters of the incident femtosecond pulse is described. Numerical calculations show that the chirped fibre grating has a broad Bragg reflection spectrum.