The invention discloses a technique for improving the breakdown voltage of an RFLDMOS. After a grid electrode is formed, a step of N2O quick thermal annealing is added before ion implantation and the forming of a drift region and trenches. According to the invention, after the etching of the grid electrode is completed and before the drift region is formed, the step of N2O quick thermal annealing is added, thereby carrying out the light doping of nitrogen between silicon and grid silicon oxide, improving the surface state between the silicon of the drift region and the silicon oxide, enabling electrons to be able to be balanced, and obtaining a more stable breakdown voltage.