An onboard Langmuir probe instrument equipped with a nonlinear micro-current acquisition device has been developed and is described in this paper. Langmuir probes are commonly used sensors for ionospheric plasma diagnosis, but the plasma density varies significantly with the altitude, latitude, and day/night; therefore, the currents collected by Langmuir probes have an extensive dynamic range and tiny amplitude. We developed a novel Langmuir probe instrument with a nonlinear micro-current acquisition device that can acquire micro-current signals with high precision and has an extended dynamic range. The instrument has a low mass (about 950 g), a power consumption of about 700 mW, and meets the payload requirements of micro-nano satellites. The instrument is equipped with high-speed analog-to-digital converters (ADCs) and a third-order low-pass filter to respectively achieve submeter spatial resolution and signal-to-noise ratio (SNR) measurement of higher than 120 dB. A data inversion algorithm based on polynomial fitting is used to accurately trace back the collected current signals to achieve high-precision diagnosis of plasma in the density range from 108 to 1013 m−3. The instrument has multiprobe acquisition capability, can run in two modes, and is suitable for different detection platforms. Various experiments were carried out in the Space Plasma Simulation Chamber to evaluate the performance of the instrument. The results demonstrate that the developed Langmuir probe instrument can accurately diagnose plasma in a large density range and meet the requirements for ionospheric plasma diagnosis.