This paper introduced chopping principle and presented a design of chopper stabilized pre-amplifier for magnetic sensor. Compared with auto-zeroing technique, chopping technique does not fold noise to DC and the noise performance is better. This chopper stabilized pre-amplifier used the integrated instrumentation amplifier AD620 as the main amplifier. It achieved low voltage noise about 10nV/iIHz, and the corner frequency of 1/f noise about 1.5mHz. Residual offset caused by nonidealities of CMOS switch was reduced by a delayed demodulate clock. And the measured result was found to agree well with the simulated result.