Digital code conversion technology between 16 kbps Continuously Variable Slope Delta (CVSD) modulation and 64 kbps A-law Pulse Code Modulation(PCM) is proposed and designed，which eliminates cumulative distortion regardless of the number of the tandem transcending. The problems about code conversion between CVSD and PCM are resolved. The whole digital code conversion of PCM-CVSD is realized based on Field Programmable Gate Array(FPGA) with Verilog language. The technology can enhance the performance of the telephone network linking between the special network and civil network.