The invention discloses a method for preparing a nanocrystalline NdFeB high-compactness magnet and relates to a method for preparing a nanocrystalline NdFeB magnet. The invention solves the problems that a bonded magnet prepared by the prior method is poor in low magnetic performance and mechanical strength and small relative density. The method comprises the following steps: breaking NdFeB casting alloy into coarse powder and carrying out the high energy ball milling of rough powder at room temperature in a hydrogen atmosphere for 10-30 hours to obtain nanocrystalline disproportionate state NdFeB alloy powder; pressing the disproportionate state NdFeB alloy powder into a high-compactness magnet blank; and then sintering under the conditions of the vacuum degree of 10<-5>-10<-2>Pa and thetemperature of 700-850 DEG C for 30 minutes to 1 hour to obtain the nanocrystalline NdFeB high-compactness magnet with the crystalline grain size of 30-80nm. The nanocrystalline NdFeB high-compactness magnet prepared by the method has the relative density of more than 0.92, the compressive strength of 212-273MPa and the magnetic energy product of 176-249k J/m<3>.