Abstract: An optical system for trapped 199Hg+ in linear ion trap was designed by the Zemax software. The optical system consists of two parts which are optical excitation system and fluorescence collection system. The optical excitation system is composed of 202Hg electrodeless lamp and excitation lens system. The diameter of 15 mm surface light which is produced by 202Hg electrodeless lamp is transformed into rectangular light of 23 mm4 mm by the excitation lens system. The rectangular light excites 199Hg+ atomic transition and then the 199Hg+ radiates fluorescence. The fluorescence is collected by the fluorescence collection system which is composed of collection lens system, filter and photomultiplier tube (PMT). The design results show that the fluorescence emitting surface is imaged well in the PMT whose diameter is 23 mm by the collection lens system. So the fluorescence collection is quite efficient and the collection efficiency is about 3%. Experimental results show that the optical system is completely satisfied for present experiment and it has a high signal to noise ratio (SNR) which is about 20. It makes necessary preparation for application of highperformance 199Hg+ microwave frequency standard.