The present application relates to a constant pressure switch apparatus and a water heating system. The constant pressure switch apparatus includes: a body inside which a flow passage is provided, the flow passage having an inlet and an outlet; a first blocking member and an attraction member that can be magnetically attracted to each other; the first blocking member having a blocking position and a communicating position that are spaced apart at a preset distance; the first blocking member being able to move towards the communicating position when a pressure difference between both sides of the first blocking member in a direction from the inlet to the outlet is greater than an attraction force between the first blocking member and the attraction member; and the first blocking member being able to move towards the blocking position when a pressure difference between both sides of the first blocking member in a direction from the inlet to the outlet is less than an attraction force between the first blocking member and the attraction member. The constant pressure switch apparatus and water heating system provided in this application can enable the circulating pump to maintain a connected state of the flow passage without increasing pressure, and can also avoid the problem of series flow of cold and hot water.