Abstract: By means of electrical tests,emission microscopy (EMMI) leakage current location and scanning electron microscope (SEM) morphology analysis, failure analysis to a 700 V vertical double-diffused metal oxide semiconductor (VDMOS) device was done. Based on the Sentaurus TCAD simulation platform to do failure verification, the main failure reason of the breakdown voltage was found. During the process of improving the termination structure, through analyzing the current density,electric field,electrostatic potential and space charge models of the termination structure, field plates(FP) problems that results in shortage of breakdown voltage were further discovered and an effective improvement was presented. Finally, after optimization, a reliable termination with a 770 V breakdown voltage and fairly uniform silicon surface electric field was obtained.