Abstract: In view of the developing problem of the valve top gap cavitation of high head single-lift ship lock, the scale 1:1 sectioning model test method which can truly describe the gap flow characteristics is adopted to study the valve top gap cavitation and the cavitation resistance mechanics of the natural aeration. It is found that three kinds of cavitation, namely the throat cavitation, the flow center cavitation and the valve plate cavitation, are taking place step by step in the gap section with the development of the cavitation. The mechanics of the natural aeration measure to prevent cavitation is that the pressure of the gap flow becomes high when the air is aerated into the gap section, and then the flow center cavitation and valve plate cavitation disappear and the throat cavitation is weakened remarkably. With the growing of the cavitation, the negative pressure zone in the gap section is gradually extending until the gap section is fulfilled with stable -10 m water head negative pressure. When the natural aeration measure is employed, the pressure in the gap section is stable in the -2 m water column and the cavitations disappear. It is nearly a quadratic polynomial relationship between the pressure of the gap section and the ventilation per meter width. The pressure of the gap is -2 m water head when the ventilation goes to the extreme value, which is the limiting equilibrium situation for the natural aeration system. When the pressure of the gap gradually increases, the ventilation volume decreases gradually until the cessation of the natural aeration. The research results are benefitcial to the cavitation resistance design of the high head valve.