This paper presents a cryogenic qubit controller for scalable superconducting quantum computing. A phase-selection digital power amplifier (DPA) topology is utilized for the XY-driving pulses generation. With a compact digital extensive architecture, the amplitude modulation and qubit phase rotation can be directly implemented at the phase-selection DPA stage for power reduction. A multi-phase envelope-tracking supply unit is also employed to enhance the power efficiency. The controller was designed and simulated in 28nm bulk CMOS technology. The controller can cover a band of 4-6GHz with an output power of at least -8dBm. The simulated SNR/SFDR is better than 55dB/46dB with a phase rotation error of less than 0.7°. The envelope-tracking supply unit can reduce the DPA power consumption by at least 24% compared to a constant power supply. The total power consumption of the controller is 3.64mW.