A 2nd order complex filter was implemented in SMIC 0.18 μm 1P4M CMOS technology. The filter’s center frequency was -1 MHz, and its bandwidth was 1 MHz. It could achieve a 10 dB image-rejection ratio. The filter employed a Tow-Thomas structure to achieve flexible gain and frequency tuning, and reduced its sensitivity to the parasitic parameters. It used an automatic tuning system based on relaxation oscillators, which could simplify the tuning circuit design and save the chip’s area and power consumption. The proposed circuit was taped out. Chip measured results showed that the chip’s area was 0.78 mm², and its supply current was 1.28 mA.