Testability is a basic design characteristic of avionics, which determines the ability of status (normal, malfunction, degradation) self-awareness and automatic internal failure isolation for avionics. Based on the whole life cycle cost, the necessity of testability design is discussed for commercial aircraft avionics. The testability design consideration of avionics general testability design, BIT design, ICs (Integrated Circuits) and components are proposed during the development of commercial aircraft. Engineering practices show that the testability design efforts can be beneficial for raising the flight safety and maintenance economy of commercial aircraft.