A 71-dB-dynamic range, 1-dB-step, low-noise programmable gain amplifier based on 0.18 μm CMOS process was presented. The current model architecture was adopted to achieve R2R common mode input range(CMIR), considering the constrained CMIR of traditional instrument amplifier with three opamps. The internal opamps’ offset voltage and low-frequency 1/f noise were reduced by chopper. The control of gain was achieved by coarse adjustment and fine adjustment, the voltage signal was converted into current signal by coarse adjustment, and the fine adjustment restored the current signal to voltage signal basing on the coarse adjustment, finally wide range and high precision were realized. The simulation results showed that the gain range of -3 dB-68 dB and step of 1 dB were totally obtained; the equivalent input noise from 1 Hz to 100 kHz at 48 dB was 3.573 μV, CMRR and PSRR at 1 kHz were 118.9 dB and 120.6 dB respectively.