High sensitivity displacement interferometer has wide applications in gravitational wave detection area, performing as crucial part in test mass dynamics measurement and seismic motion monitoring for low-noise observatory operation. With advances in heterodyne laser interferometry, sensitivities at levels of sub-nm/šHz over sub-Hz frequencies can be achieved. However, the breakthrough towards picometer level still needs various techniques in noise characterization and suppression. In this article, a compact heterodyne laser interferometer design as well as benchtop prototype system is presented. Common noise sources and their effects are investigated, including laser frequency noise, non-linear OPD noise, thermo-elastic noise, as well as readout noise from phasemeters and photoreceivers to determine the sensitivity limits in our system. Furthermore, each individual noise source is characterized with dedicated instruments and the coupling coefficients are determined respectively. By subtracting the individual noise contributions, the interferometer sensitivity reaches a sensitivity at the picometer level above 100 mHz frequency. We will present our progress and current results.