In order to optimize the surface electric field of AlGaN/GaN high electron mobility transistors (HEMTs), a novel AlGaN/GaN HEMT has been grown with a step AlGaN layer, made for the first time as far as we know, to improve the breakdown voltage. The discipline of the 2DEG concentration varying with the thickness of the AlGaN epitaxy layer has been applied to the new AlGaN/GaN HEMTs with AlGaN/GaN heterostructure. By thinning the AlGaN layer near the gate edge, the 2DEG concentration in the channel is made to form the low concentration region near the gate edge. New electric field peak has appeared at the corner of the step AlGaN layer. The high electric field has been decreased effectively due to the emergence of new electric field peak; this optimizes the surface electric field of the new AlGaN/GaN HEMTs. Then the breakdown voltage is improved to 640 V in the new AlGaN/GaN HEMTs with the step AlGaN layer as compared with 446 V for the conventional structure. In order to let the breakdown curve consistent with the test results, a certain concentration of the acceptor-like traps is added to the GaN buffer to capture the leaking current coming from the source electrode. Simulation results verify the causes for doping acceptor type ions to the GaN buffer, given by foreign researchers. The breakdown curves have been obtained which are consistent with the test results in this paper.