The Fraunhofer far-field diffraction theory and a sectorial screen were used to study the low-order femtosecond vortex beams, theoretically and experimentally. The experimental results show that the frequency component within femtosecond laser can produce destructive interference in the high-frequency area. And the more frequency component is, the stronger the effect becomes. However, it can't change the principal maximum part. So, the sectorial screen can also be used to detect the femtosecond vortex beams. The results also show that the uniformity of the energy distribution can be affected by angles of the sectorial screen. It decreases with the increase of angle. The sectorial screen method was extended to detect the femtosecond vortex beams in this work. It provides a basis for the selection of the best measurement angle. Moreover, the method could be the theoretical and experimental basis for the study on vortex beams from femtosecond amplifiers.