Superconducting qubits are promising candidates for building scalable quantum computers, but they require efficient and flexible readout systems that operate at cryogenic temperatures. This article presents a cryo-CMOS analog baseband (ABB) for superconducting qubits readout, which targets flexibly reconfigurable gain, center frequency, and bandwidth. The ABB achieves power and area reduction by integrating programmable-gain amplifiers and filters in one block and by using differential capacitors rather than single-ended ones. The differential-mode gain and common-mode stability of the low-pass filter are improved by negative-impedance inverters. Implemented in 28-nm Bulk CMOS technology, the ABB occupies an active area of 0.016 mm2. It has a tunable gain range from 10.7 to 42.5 dB, a center frequency from 101 to 189 MHz, and a bandwidth from 1.3 to 2.8 MHz. Thus, the ABB enables high-fidelity and low-power readout of superconducting qubits for future large-scale quantum computers.