This paper introduces the primary purpose,a series of characteristics and coating methods of crystal optical coatings in high power YAG lasers and solid middle infrared lasers.The coating-designs with low loss and high reliability have been developed based on the using of computer coating optimization software.In order to explain main technique problems of these coatings on laser crystal in details,such as deliquescence,absorption and contamination,we discussed coating process and films on different working-facies,taken YAG slab and ZnGeP2 as examples.According to the contrastive results of characteristic analysis and laser damage threshold,it has been found that ion assistance deposition technique increase the packed density of films,enhances the adhesion and obviously improves mechanical strength and environmental stability.Presently,these coatings of laser crystal have been used in high power YAG lasers and mid-IR solid lasers.The effects are 16 kW/cm2(slab)and 10 W output at 5 kHz(ZnGeP2).