A method for preparing an aluminum-oxide single-layer coating or multi-layer coating on the surface of silicon-nitride ceamic cutting tool by using a Physical Vapor Depositon (PVD) process. There are two methods for preparing the aluminum-oxdie coating by using the PVD techonology: one is depositing an aluminum-oxide layer through reactive sputtering of Al target by using a bipolar pulse power source and a Dual Magnetron Sputtering (DMS) technology in the O 2 atmosphere or Ar-O 2 mixed atmosphere; the other one is sputtering, by using DMS technology in the Ar atmosphere or Ar-O 2 mixed atmosphere, an aluminum-oxide ceramic target conductive doping, and then preparing the aluminum-oxide coating. With the two methods, a single-layer aluminum-oxide coating, a nitride and aluminum-oxide composite coating, or a multi-layer or nano composite coating obtained by alternately depositing nitride and alumimum oxide is prepared, the aluminum-oxide being ±-Al 2 O 3 , ²-Al 2 O 3 , ³-Al 2 O 3 , or amorphous Al 2 O 3 . The hard nitride coating in the multi-layer composite aluminum-oxide coating comprises a two-element nitride coating, a three-element nitride coating, multi-element nitride coating, or a multi-layer composite thereof of a nano coating.