A stable beam flow intensity modulation device of a circular accelerator, comprising a first electrostatic deflection device (1), a second electrostatic deflection device (2) and a vertical flow-limiting gap (3), wherein the first electrostatic deflection device (1) and the second electrostatic deflection device (2) have the same structure and respectively comprise an upper pole plate and a lower pole plate which are vertically symmetric about a central plane of the circular accelerator; the vertical flow-limiting gap (3) comprises: a pair of plates, having L-shaped cross sections and being vertically symmetric around the central plane; the first electrostatic deflection device (1), the second electrostatic deflection device (2) and the vertical flow-limiting gap (3) are respectively arranged on upper surfaces and lower surfaces of three peak region magnetic poles in the circular accelerator; a beam flow generates a vertical rail projection by the combined effect of the first electrostatic deflection device (1) and the second electrostatic deflection device (2); the vertical flow-limiting gap (3) is used for blocking a beam flow which has relatively large vertical deflection; and the beam flow intensity is adjusted by changing the size of a vertical protruding rail of the beam flow and adjusting a beam scraping proportion. The vertical motion of a beam flow is prevented from becoming unstable by correcting a scraped beam flow back to the central plane, so that the intensity of a lead-out beam flow is stably adjusted.