Abstract: Aiming at that the π/4-DQPSK digital modulation need large amount of operation and fast modulation is achieved difficultly. A fast arithmetic based on digital modulation is present in this paper. The arithmetic bases on TMS320C54x DSP processor. Difference coding and signal mapping is completed by look-up tables, Shaping FIR filtering for channel I and Q is completed by wave storage and Carrier modulation is completed by strobe and reading directly. There are no multiplications in whole modulation. The speed of modulation is faster. Simulating result proves the effectivity of the arithmetic.