AbstractMagnetotelluric (MT) sounding is an important geophysical method to probe crust and upper mantle structure. The long‐period MT instrument is essential during the project “Deep Exploration Technology and Experimentation (SinoProbe)” in China. However, China cannot produce the long‐period MT instrument by itself now and only relies on LEMI‐417 imported from Ukraine. When using this instrument, we find the resolution of telluric field meter is 0.01 mV/km and the resolution of magnetometer is 0.01 nT through its data file Final.asc, which shows its accuracy is low compared with other MT instruments. And the LEMI‐417 did not work normally without GPS signal when data acquisition using LEMI‐417 was conducted by our group team. Therefore, it is necessary to develop a set of long‐period MT instrument which meets the need of our project. As MT signals, include telluric field signal and magneto signal, are weak and easy to be embedded in noise, they need to be amplified. Thus, the amplifier CS3301 with low noise and least distortion of harmonic waves is selected. The CS3301 provides four input choices and seven gain selections, which are made through using the MUX and GAIN pins. After analysis and testing, CS3301 is set under Gainx64 and input mode 2 in the telluric field signal acquisition circuit. In this case, if the polar distance exceeds 100 meter, the resolution of telluric field meter is 0.001 mV/km. It is set under Gain×64 and input mode 3 in the magneto signal acquisition circuit, which uses the feedback method to deduct the constant signal and makes the amplifier only amplify variable signals. And in this case, the resolution of magneto meter is 0.006 nT. As comparison tests, the instrument with CS3301 and LEMI‐417 were used to collect MT data at several measuring points in the same time, such as measuring point 9005 in south China and measuring point 1600 in northeast China. The results show that their time series curves are consistent and the MT sounding curves after data processing are also consistent, indicative of success of the instrument design.