The BBO crystal has insensitivity to the wavelength near 1550 nm where mode-locked Er-fiber lasers operate through the analysis of the Sellmeier equations, which can be used for broadband frequency doubling of femtosecond Er-fiber laser. Split-step Fourier method is used to numerically solve the coupled wave equations of frequency conversion. The conversion efficiency and the output spectral bandwidth with the length of the crystal under different power densities and different durations of the fundamental pulse are obtained. On this basis, citations are made to select the best size of BBO crystal. Compared with the small signal approximation, this method is applicable more widely, which can be used to choose the crystal under different power densities.