The TianQin project plans to send three satellites into Earth orbit, forming a triangular array with sides about 170,000 km long. Laser interferometers will be set up to measure the relative motions of the test masses. The overall displacement resolution of the optical metrology system needs to achieve 1 pm/Hz1/2@6 mHz. The performance of the phasemeters extracting the interferometric phases is one of the baselines to fulfill the requirement of the accuracy of the displacement measurement. In this paper the core metrology of the phasemeter is introduced and the performance of the 4-channel phasemeter is tested using signal generator. After improving temperature stabilities around the phasemeter and the signal generator, the differential phase noise of the phasemeter prototype achieved 1 μrad/Hz1/2@6 mHz.