Miniaturization and portability are the targets in the design of the aeronautic testing instruments. For the advantages of PC104 bus system such as its small volume and compact structure ,the principles and the method of PC104 bus technology are applied during the design for testing circuit in an in-field testing instrument of an aeronautic equipment. And the problems such as anti-jamming and the bus conflict are solved successfully in the engineering practice. The design results show that the designed testing instrument has the strong points of small volume , lightweight and exact testing result.