Aiming at the positioning platform of heterodyne interferometer, a new position method of phase locked has been introduced to simplify the system structure. The positioning system working principle and process have been explained and the feasibility of the phase locked method has been proved. The core link of positioning system, such as high-frequency, phase-shifting circuit and phase demodulation unit, have been discussed in detail. The test experiment of phase demodulation circuit and the whole positioning platform have been performed. The results show that the precision of phase demodulation circuit is 0.07o and the positioning uncertainty is less than 10.1 nm.