In GRENOUILLE femtosecond pulse measurement system,time delay is a key parameter,which directly determines measurement range of femtosecond pulse width. The relationship among the time delay,Fresnel biprism’s base angle,the input pulse’s spot diameter was quantitatively analyzed. The research results show that spot radius,the distance between Fresnel biprism and thick SHG crystal and the maximum time delay will become greater when base angle of Fresnel biprism becomes bigger,and thus the measured femtosecond pulse will become wider. The device parameters of the optical system are determined and verified by experiment,the error of the measurement is 4.8%,which provide a reference for the optimization and design of GRENOUILLE measurement system.