A bit allocation method and apparatus for a multicarrier modulation signal and a system where the method includes: determining a signal to noise ratio margin according to a predefined probability of a maximum clipping noise that can be allowed by the system; and allocating the number of modulating bits and power in each subcarrier for a multicarrier modulation signal according to the signal to noise ratio margin. By presetting a signal to noise ratio margin for a clipping noise, tolerance of the signal for the clipping noise is increased, and bit error rate is efficiently lowered.