Embodiments of the present invention provide a method for transmitting low rate signals over an optical transport network, including: adapting the low rate signals into low rate optical channel data units of the same rate level with the low rate signals; asynchronously mapping each of the low rate optical channel data units into a low rate optical channel data tributary unit respectively, and generating justification overhead used for rate adaptation for each of the low rate optical channel data units; and forming a higher order optical channel data unit with at least one low rate optical channel data tributary unit and justification overhead corresponding to the low rate optical channel data tributary unit. The present invention enables the optical transport network to support mapping, multiplexing and highly efficient transmission of low rate signals.