The fault restoration technique based on n:m scheme can improve system availability with small cost increases for WDM networks, where it is possible that n faults occur synchronously in a protection domain or a protection entity, but it has rarely been implemented or standardized. In this paper, an analytical model for mean restoration time of restoration mechanism based on n:m scheme is developed. As a general model, it aims at the WDM networks with the restoration paths created in advance of fault occurrence without wavelength reserved. In particular, the proposed protocol for processing faults has been improved to make the restoration based on n:m scheme as fast as that based on 1:1 or n:1 scheme.