High energy acceleration of the hydrogen negative ions has been investigated using a 350 keV negative ion source. The source consists of a cesium seeded multicusp plasma generator, an extractor and a two‐stage electrostatic accelerator. Beams of 350 keV, 130 mA, 1 s were obtained stably with a current density of 9.4 mA/cm2. The heat loadings of the accelerator grids caused by the electrons leaking from the extraction gap to the acceleration gaps were negligible. The heat loadings by the direct interception of the negative ions were reduced to a few percent of the acceleration power by optimizing beam optics. The heat loadings by the stripped electrons from the negative ions were dominant and proportional to the source operating pressure. Therefore, it is important to operate the source at as low a pressure as possible.