The aim of the research is to design a permanent magnet generator with rated power of 300 kW and rated speed of 2 700 r/min. For this generator, the center height should be no more than 225 mm, and the power density is about 6 times that of ordinary generator. Therefore, the temperature rise control of the generator has become an urgent issue. In order to reduce the temperature rise of the generator, the water cooling frame is designed. Through the finite element simulation calculation, it is found that the temperature rise at the end of the generator winding is still high. In order to further reduce the winding end temperature rise, four axial ventilation channels are added on the basis of the raw water cooler base, and the heat exchange between the cooling water and the cooling gas in the four axial ventilation channels is made possible. In this way, the calculated temperature rise of the generator end is significantly reduced. In order to verify the feasibility of the hybrid cooling structure, a prototype is made and type test is carried out. The test results are basically consistent with the design values, which verifies the accuracy of the design and the effectiveness of the axial ventilation channels for heat dissipation of the end windings.