The interference behavior of high-l Rydberg states is investigated in external fields. We prepare high-l states from an initial excited ns Cs Rydberg state by applying one electric-field pulse. The interference between the initial ns state and the high-l states is investigated by two time-delayed short electric-field pulses. The state selective field ionization technique is used to measure the transfer ratio versus the field pulses parameters. The visibility of interference is defined to describe interference and the results show that a relative long duration of electric-field pulse will weaken the interference.