To research the electromagnetic environment of ultra high-voltage direct-current（HVDC） transmission lines

we developed a wireless electric field measuring instrument. Moreover

we selected 30 prototypes to test the electromagnetic compatibility（EMC） performance of electric measuring instrument

and analyzed the test results of electric measuring instrument’s EMC performance in detail. Combining electromagnetic working environment with interference routes of the electric measuring instrument

some important factors which were susceptible to electromagnetic interference（EMI） in testing and the corresponding improvement measures were proposed. Test results show that the most obvious faults occur in the radiated

radio-frequency

electromagnetic field immunity test and surge immunity test. But the faults of electromagnetic radiation and surge harassment can be conquered by increasing the thickness of the sheet copper inside the equipment and adding transient suppression diodes and filter at the device power supply port.