A smart radiation device (SRD) of VO2/Si3N4/Al structure with tunable emittance is designed. The optical properties of SRD are studied through the optical thin film characteristic matrix analytical method. The results show that emission properties of SRD are determined by the thickness of VO2 and the modulation wavelength range is related to the thickness of Si3N4 dielectric layer. The emittance tunability of the optimized SRD is 0.38. The SRD with MgF2/Si3N4 bi-layer anti-reflection coating presents an emittance of 0.30 at 20 ℃ and 0.91 at 100 ℃, and the tunability is increased to 0.61. The finite difference time domain method is used to analyze the modulated radiation distribution of SRDs with or without anti-reflection coating. It is proved that the added anti-reflection coating can improve the emittance-switching efficiency of SRD and enhance its adaptability in complex environment. The method can give reference to the design of high performance SRD and meets the need for a lighter thermal control device of spacecraft.