Abstract: In DSP, restricted by the calculating precision of some digital algorithms, the digital signal transition from high quantizing precision to low quantizing precision was necessary, i.e. the truncating of bit width of signal. The direct truncating operation on signal of high quantizing precision led to the decline of the spurious free dynamic range (SFDR) of truncated signal. Truncating after adding Dither signal to the signal of high quantizing precision could decline the harmonic distortion of the truncation error efficiently and extend the dynamic range of the truncated signal significantly. The simulation results of MATLAB shown that truncating after adding Dither signal could extend the SFDR of the truncated signal by about 16dB.