The invention discloses a manufacturing method of an MOS capacitor, mainly solving the problem of overly high interface trap density of SiC/SiO2. The manufacturing process comprises the following steps of: after washing and cleaning epitaxy material of N-SiC, dry-oxygen oxidizing a layer of SiO2; orderly finishing annealing in Ar gas environment, wet oxidization and annealing in wet oxygen environment and cooling treatment in Ar gas environment to an oxidized sample piece; adopting chemical vapor deposition to deposit one layer of SiO2 with the thickness of 25nm to 85nm on the sample piece after cooling treatment again; in the SiO layer, ion-injecting N<+> to the interface of SiC/SiO2, and annealing in Ar gas environment; and through a photolithography mask, vacuum sputtering A1 as an electrode and annealing in Ar gas environment to finish the making of a capacitance component. The manufacturing method has the advantages of precise control of the dose of N<+>, low trap density of SiC/SiO2 interface, and compatibility with the existing technique, and can be used for improving SiC/SiO2 interface characteristics of N-type SiC MOS devices.