A wavelength domain simulation method for wavelength-division-multiplexed (WDM) all optical network is presented. The optical spectra of WDM channels through 3 cascaded optical add-drop multiplexers (OADM′s) are simulated, and compared with experimental results. It is shown that after the WDM signals propagate through 50 OADM′s, the optical signal noise ratio (SNR) is above the acceptable level necessary to achieve a bit-error-rate (BER) lower than 10^-9at 10 Gb/s. These simulations indicate that a future national or regional scale transparent WDM network is feasible.