Study on picometer-level laser interferometer readout system in TianQin project

•To obtain the accurate correction coefficient, a new pilot tone correction method based on LSM is also proposed to evaluate experimental results.•After pilot tone correction, the phase fluctuation is mostly less than 10 µrad.•The correlation between the signal corrected before and after by LSM method is less than half of that by NPT method.•The noise floor of the phasemeter was about 5 µrad/√Hz, meets the TianQin requirement of 1 pm/√Hz.