The invention discloses a direct digital frequency synthesizer, a frequency synthesizing method thereof and a frequency modulation transmission device. The direct digital frequency synthesizing method includes: acquiring work clock frequency and a frequency control word; determining the target frequency dividing ratio corresponding to a frequency error satisfying preset requirements, a lookup table length and a phase stepping value according to the maximum length value of a storage, the work clock frequency and the frequency control word; generating a lookup table according to the lookup table length and the phase stepping value; generating a triggering signal according to the work clock frequency and the target frequency dividing ratio; acquiring an amplitude, corresponding to a count value, from the lookup table according to the count value corresponding to the triggering signal, and performing digital-to-analog conversion on the amplitude to output a sine-wave signal. By the direct digital frequency synthesizing method, the length of the lookup table can be reduced effectively.