The theory of secondary spectrum which exists in long focal-wide bands optical systems is introduced in this paper and the qualifications needed to eliminate are also analyzed.BOL devices and ordinary optical glass have different Abbe number and relative aberration.By chiseling a BOL on a surface of a lens,a apochromatic system is composed,which actualizes the system′s apochromatism,as well as revising primary aberration.Experimental results show that the system is characteristic of simple structure and light weight and can be used in mini-long focal telescope systems.