The invention relates to an anti-collapse plugging agent for drilling fluids, and a preparation method thereof, and a drilling fluid, and belongs to the technical field of oil field drilling fluids. According to the present invention, the anti-collapse plugging agent is selected from a chitosan nano-gel, wherein the chitosan nano-gel has an average particle size (D50) of 50-500 nm, has a zeta potential of 0 to +50 mV, and is prepared by a method selected from a chemical cross-linking method, an ion gel method, a precipitation method and an emulsion drop aggregation method; the use amount of the anti-collapse plugging agent for drilling fluids is 0.1-5 parts by weight, preferably 1-2 parts by weight, by using the water in a drilling fluid as 100 parts by weight; the chitosan nano-gel micro-particles can be deformed, and are positively charged on the surface, such that the chitosan nano-gel micro-particles are easily squeezed into the micro-nano pores and micro-cracks of mud shale underthe action of downhole pressure difference so as to provide the effective anti-collapse plugging effect; and the drilling fluid can be used in various oil-gas wells such as straight wells, directionalwells, large displacement wells, horizontal wells and the like.