A 4-6GHz cryogenic radio frequency digital-to-analog converter (RF-DAC) is presented for quantum computing application in this brief. The RF-DAC is implemented with a switched-capacitance topology for its characteristic of being insensitive to cryogenic temperature. A cross-connection switched capacitor array is also integrated into the RF-DAC to enhance the power on/off ratio. The cryogenic characteristics of a passive transformer are discussed in detail with a lumped model and accuracy EM simulation method. Fabricated in a 28nm CMOS process and verified at 3.5K, the RF-DAC can generate the driving pulse correctly with a sampling clock frequency of up to 2GS/s. The achieved SNDR and power on/off ratio are better than 41.3dB and 36dB, respectively. The output power of the RF-DAC is >-13dBm over a band of 4-6GHz.